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close this bookSourcebook of Alternative Technologies for Freshwater Augmentation in some Asian Countries (UNEP/IETC, 1998, 192 pages)
close this folderPart B - Technology profiles
close this folder2. Wastewater treatment and reuse technologies
View the document2.1 Sewage reclamation using conventional wastewater treatment
View the document2.2 Sewage reclamation using reverse osmosis
View the document2.3 Wastewater treatment using wetlands
View the document2.4 Wastewater treatment using duckweed
View the document2.5 Wastewater treatment using lagoons
View the document2.6 Other technologies of wastewater treatment and reuse

2.1 Sewage reclamation using conventional wastewater treatment

Technical description

In India, treated municipal sewage is being used by industry for cooling water and in firefighting. The Hindustan Petroleum Company Limited (HPCL), located in Bombay, has used seawater for these purposes, but is in the process of converting to the use of reclaimed water (see the Indian Case Studies in Part C of this Source Book). The use of reclaimed water better meets the pollution control regulations established by the Central Pollution Control Board's MINAS (Minimal National Standard) regulations, and minimizes the operation and maintenance problems inherent in the use of the seawater for cooling and firefighting purposes. Use of reclaimed sewage also enables the refinery to reduce the number of blow downs from the cooling towers and thereby reduce the volume of cooling water effluent required to be treated to MINAS.

The components of the sewage reclamation, treatment and reuse process are a sewage pump housed in a dry well for easy access, a rising main to convey wastewater from the pump house to the factory premises, a water reclamation plant within the factory, a storage reservoir for the reclaimed water, and a distribution system to channel the reclaimed water to the cooling towers. A wet well is likely to be required to retain the incoming sewage from the sewer and balance flows to the treatment plant, and a mechanical screen should be installed in the system upstream of the pump to remove particulates prior to the wastewater entering the pump house.

The reclamation plant typically consists of a flash mixer for mixing of chemicals with the incoming wastewater. An alum solution, used to flocculate particulates that have passed through the mechanical screen, is dosed in a chamber upstream of the flash mixer. Following dosing. The wastewater passes through a clari-flocculator to remove fine suspended matter and colloidal turbidity. The resultant clear liquid flows over a weir and is collected in the launder. The resultant sludge is collected in the bottom of the clarifier tanks and discharged via the excess sludge sump. The clarified wastewater is then filtered through a rapid sand filtration unit, using a layer of quartz sand and a layer of graded gravel, and separated into two streams. One stream is passed through an ion-exchange softener unit, prior to being recombined with effluent stream in proportions calculated to produce the desired degree of hardness. The blended, reclaimed water is then chlorinated using a vacuum type chlorinator.

Extent of Use

This technology can be used in industries where a large volume of cooling water is required and an adequate source of wastewater is readily available.

Operation and Maintenance

Maintenance is related to the operation of the water reclamation plant and pumping system. Operations are generally conducted over a 24 hour period, requiring adequate trained human resources in at least three shifts to operate the treatment plant.

Level of Involvement

This technology may be implemented at the individual industry level or incorporated into a local government wastewater treatment scheme.

Costs

The total capital cost of a 15 million litre per day (MLD) reclamation system is about $ 4 million. The annual operation and maintenance costs are about $410 000, or about $ 0.02 per m3.

Effectiveness of the Technology

Use of reclaimed water is expected to reduce the cooling makeup water requirement from 4 500 m3/hr (or 108 000 m3/day) to about 625 m3/hr (or 15 000 m3/day).

Suitability

This technology is suitable where large quantities of wastewater are available nearby. Advantages

Use of this technology reduces the problem of high TDS in the cooling water which occurs when sea water is used as cooling water. Where municipal water is used for cooling purposes, use of reclaimed wastewater also results in a net savings in the drinking water supply of a municipality since industrial demands on this source are reduced.

Disadvantages

Domestic wastewater is best suited for reclamation as industrial wastes may contain contaminants that make such wastes unsuitable for reclamation. This technology has an high capital cost, especially if the sewage line is far away from the industry, and may have relatively high operation and maintenance costs, depending on the reclamation technology used.

Cultural Acceptability

No problems have been noted since the reclaimed water is not for human consumption.

Further Development of the Technology

The technology is readily transferable and can be used by other industries. New industries should consider the use of reclaimed water in their overall plan, which will make it cost effective to implement.

Information Sources

AIC Watson,
The Sewage Renovation Project at Hindustan Petroleum Corporation Limited (HPCL), Bombay, India. AIC Watson.

2.2 Sewage reclamation using reverse osmosis

Technical description

Industries in growing metropolitan areas may face production losses as a result of excess demand for municipal water. Madras Fertilizers Limited (MFL), Madras City, Tamil Nadu, India, has faced such a situation in 1983 and 1987 (see the Indian Case Study in Part C of this Source Book). As a result, MFL has explored alternatives including the use of desalinated sea water and treated wastewater to supply process and cooling water to its operations. After detailed review of these alternatives, the Company decided to reclaim water from city sewage using advanced waste water treatment followed by Reverse Osmosis (RO) as an additional purification step. The Company has a daily water requirement of 20.25 MLD, 68% of which is required for cooling purposes.

Wastewater used by the plant is treated to tertiary standards using an activated sludge process, with the treated water being further reclaimed through excess lime addition, ammonia stripping, recarbonation, chlorination, multimedia filtration, activated carbon filtration, cartridge filtration, and reverse osmosis using thin film polyamide membrane.

Ammonia stripping is carried out in first-stage and second-stage counter current flow ammonia strippers, which are similar to cooling tower cells. Treated wastewater is sprayed from the top while air is sucked in from the bottom of the tower by an induced draft (ID) fan located at top of the tower. Free ammonia is blown out of water into the air. The ammonia-stripped water is pumped to a first stage carbonation tower and calcium carbonate clarifier, where the pH is brought drown to 7.0, and chlorinated before being sent to storage. The excess sludge from the clarifier is disposed of in sludge beds, and water drained from sludge is recirculated into the inlet lagoon.

Although most undesirable constituents like BOD, hardness, and ammonia are removed by tertiary treatment, the total dissolved solids (TDS) content is generally higher than well water. This would increase overall water consumption by making it necessary to add make up water regularly to dilute the salinity, increase the corrosiveness of the recirculating water, and increase chemical dosing needed to keep corrosion and sealing problems in check; all of which result in increased operating costs. To reduce these undesirable salinity-related costs, MFL selected Reverse Osmosis (RO) treatment of the treated effluent as a convenient and viable method.

Extent of Use

Industries requiring large volumes of cooling water could use this technology.

Operation and Maintenance

A qualified chemical engineer is required to supervise the treatment process. Other operation and maintenance requirements include the maintenance of the physical facilities, routine monitoring of the plant operation, and oversight of the supply and circulation system.

Level of Involvement

This technology is typically implemented at the individual industry level.

Costs

The total capital cost of a 20 MLD reclamation and reuse facility is estimated to be about $ 18 million. Annual operation and maintenance costs are about 10% of the capital cost.

Effectiveness of the Technology

Reclaiming sewage releases an equivalent amount of potable water in the municipal water system for domestic and other uses in the city.

Suitability

This technology is suitable for use in areas where a large quantity of sewage water is available nearby.

Advantages

Use of reverse osmosis proved to be a less expensive alternative than other alternatives such as sea water desalination, and resulted in a savings in the drinking water supply.

Disadvantages

The initial capital cost of an RO system may be high, especially if the sewerage line is far away from the industry. This system is also expensive to operate due to high power consumption requirements.

Cultural Acceptability

No problems are known as the reclaimed water is not for human consumption,

Further Development of the Technology

The technology is transferable and can be used by other industries. New industries should consider integrating this technology into their overall plant design to make it cost effective.

Information Sources

Rajappa, M.S. 1990. Reclaimed City Sewage as Industrial Water. Journal of Indian Water Works Association, Jan-March, 95-100,

2.3 Wastewater treatment using wetlands

Technical description

Untreated wastewater is usually discharged into nearby streams or water courses. It is generally assumed that the waste assimilative capacities of these natural water sources are high and can be sustained in the long term. However, as the negative effects of this waste disposal philosophy are increasing, low cost and low energy alternative systems, such as utilization of nearby wetlands, is usually indicated. Wetlands which lie in the buffer zone between the municipal areas, agricultural fields and the water courses provide a sound means for filtering wastewater before it is discharged into a river or other surface water feature. In the past, natural wetlands have been used as natural nutrient sinks for the treatment of wastewater.

Wetlands act as natural purification systems. Their hydrological regimes, sediments, and biotic components enhance the ability of wetlands to process wastewaters. Hydrological regimes are influenced by precipitation, surface water inflows, groundwater inflows, evapotranspiration, surface water outflows, groundwater outflows and changes in the water storage capacity of the system. Wetland sediments accrete carbon through decomposition of organic matter. This may result in very low oxygen concentrations within sediments. The systems exhibit very high primary production rates with the resulting organic soils having low bulk densities, high water holding capacities, low hydraulic conductivities, high organic matter contents, and extremely high caution exchange capacities (Eassan et al., 1988), retaining most of particulate organic matter produced in the wetland. Biotic components include plants, phytoplankton, invertebrates and vertebrates.

Operation and Maintenance

Operation and maintenance requirements depend on the type of the reclamation system. Pumps require monitoring and a preventive maintenance system, which requires skilled personnel, especially if there are several pumps within the system. Periodic inspections and ecological monitoring are required to ensure the quality of the output water, and to maintain the wetland vegetation.

Level of Involvement

This technology may be implemented by government agencies and communities.

Costs

The capital costs of constructing and managing a wetland treatment system vary widely according to specific local conditions. In augmented natural wetland systems, the capital costs consist solely of the cost of pipes and pumps. In constructed wetlands, land acquisition and development costs are also incurred. Easson et al. (1988), citing Tuchobanoglous and Culp (1980), provide a general guideline to the capital costs of wetland wastewater treatment, in 1980 dollars, as shown in Table 7. Costs of wetland treatment could be lower in Asia. The per unit cost of wetland treatment of wastewater, as provided by Easson et al. (1988) citing Fritz and Helle (1979), was one-half of that of a conventional treatment system. The operation and maintenance costs are also comparatively low, as wetland treatment systems require only periodic inspection and ecological monitoring. Nevertheless, the environmental investigations needed to identify the linkages between ecosystem components in the case of augmented natural wetland systems may increase the cost of implementing this technology significantly.

TABLE 7. Cost of Wetland Treatment Systems

Costs

Plant size (m3/day)


380

1900

3800

Capital costs





Land requirement (ha)

1.6

8

16


Capital cost (million $)

0.49

1.12

1.18


excluding land costs





Amortized capital ($)

49905

114072

183 330

Operation and maintenance costs





Labour ($10/h)

12500

30000

45000


Power, 50-60/kWh

5883

11494

18600


Parts and supplies

3500

4500

6500


Total Operation and maintenance

21883

45994

70100

Total Cost ($)

71788

160066

253 430

Unit cost ($/m3/yr)

0.52

0.23

0.18

Effectiveness of the Technology

Biological treatment of wastewater by wetlands has been found to reduce the levels of virtually all contaminants, including those present in wastewater from mines (Fenessy and Mitsch, 1991). Wetlands are effective in reducing, by up to 90%, the concentrations of nitrogen, pathogenic bacteria and heavy metals in wastewater (Easson et al., 1988, citing Rogers et al., 1985). System performance, however, is determined by various factors, including water depth, temperature, pH, and dissolved oxygen concentrations, and by the type of wetland constructed or considered for use in wastewater treatment. Natural wetlands include shallow and deep water marshes, mangrove swamps, cypress domes, tidal marshes, bogs, and peatlands. Constructed wetlands may be artificially to reflect this diversity of wetland types.

Suitability

The suitability of wetland treatment systems for wastewater management depends on a wide range of conditions. Generally, large wetlands are more suitable for use as a treatment system because of their larger surface area, greater number and variety of aquatic plants and reduced susceptibility to flooding when wastewater is applied at a rate likely to be generated by a small municipality. Larger wetlands are also more likely to be able to treat wastewater on a year round basis. Smaller wetlands may become costly in the absence of mechanisms to control the rate and volume of wastewater applications.

Advantages

Wetland systems have several distinct advantages. Natural wetlands are immediately available without further need significant for the construction of facilities. In the case of constructed wetlands, wetland treatment systems also help to create additional wetland habitat. Wetlands may also provide an opportunity for partial cost recovery through the harvest of peat or vegetation for the use in the manufacture of pulp, compost, food for livestock, or vegetative material for biogas production.

Disadvantages

Disadvantages of the systems include climatic limitations on the active growth phase of the wetland vegetation and the land area required. A cold climate can become a limiting factor for the adoption of such a technology. The technology also requires relatively large areas which may not be readily available near cities or towns. Further, wetlands can produce nuisance insects. In cases where little is known of the relationships between various biotic and abiotic components of a wetland, the effects of using the wetland for water quality management purposes on the overall ecosystem may not be readily apparent.

Cultural Aspects

Health risks, along with other cultural barriers, make it difficult for the widespread adoption of wetland treatment technologies for wastewater treatment and reuse; people feel uneasy using wetlands where wastewater is treated for other economic purposes such as harvesting of vegetation or peat.

Further Development of the Technology

There is an high potential for the further development of wastewater treatment systems based upon wetlands in many parts of Asia. This potential can contribute to the reuse of wastewater in those areas where there is a growing demand for water. However, a cost effective means of pretreating wastewater to reduce pollution levels prior to discharging it to wetlands should be found.

References

Asian Institute of Technology (AIT) 1992. Sewage Purification Through Aquatic Plants, Final report. Division of Environmental Engineering, AIT, Bangkok.

Easson, M.E. et al. 1988. Sanitation Technologies for Cold/Temperate Climate. Environmental Sanitation Review, No. 25, AIT, Bangkok.

Fennessey, M.S. and W.J. Mitsch 1989. Design and Use of Wetlands for Renovation of Drainage from Coal Mines. In: Ecological Engineering: An Introduction to Ecotechnology, W.J. Mitsch and S.E.Jorgensen (eds), John Willey and Sons, New York.

2.4 Wastewater treatment using duckweed

Technical description

This is a relatively new technology in which small-scale wastewater treatment can be achieved using duckweed (Lemna spp. or Spirodela sp.). Duckweed is a self growing plant abundant in the tropical countries. It is commonly used as a fertilizer in paddy fields, but has recently been used in the treatment of wastewater in Bangladesh. In Mirzapur, Bangladesh, this technology has been implemented at the village level as part of a UNDP project examining the potential of duckweed-based wastewater treatment and fish production.

Operation and Maintenance

Use of this technology is simple, being based upon a modification of conventional maturation lagoon technology (see Wastewater Treatment Using Lagoons below). Maintenance consists of removal of excess biomass to encourage continued growth of the duckweed community, and thereby removal of nutrients from the wastewater, and maintenance of the containment structure of the pond.

Level of Involvement

This technology can be implemented at either the individual farm or community levels.

Costs

No data are available, but costs are estimated to be low.

Effectiveness of the Technology

Since 1989, PRISM, Bangladesh, has developed farming systems using duckweed-based technology and tested their potential for wastewater treatment and fish food. The results have been promising and, together with similar activities in Lima, Peru, have succeeded in generating interest among multilateral as well as bilateral donors in further examining the potential of this technology.

Suitability

This technology is suitable in tropical climates.

Advantages

This technology is inexpensive to construct and operate, and easy to implement. Duckweed is a prolific plant, especially in nitrogen-rich environments, and can be easily used as mulch or a natural soil organic enrichment.

Disadvantages

If the flows through the oxidation pond are not properly controlled, there is a possibility that the duckweed will flow out with the effluent. Treatment capacity may also be lost during high floods, if the area is not protected.

Cultural Aspects

No problems relating to the use of this technology are known to occur.

Further Development of the Technology

More research through pilot projects is needed in order to refine the sizing of the ponds used and to determine the correct innocculum of plant material to achieve a predetermined effluent quality.

Information Sources

Gert van Sanden, EMTAG, INUWS. The World Bank, 1818 H Street NW, Washington DC.

Jan van der Laan, DGIS, Royal Netherlands Embassy, New Delhi, India.

Erik S. Jensen, Danida, Royal Danish Embassy, Road 51, Gulshan, Dhaka, Bangladesh, Tel. 880 2 881799, Fax 880 2 883638.

Mohammed Ikramullah, PRISM, Bangladesh, House 67, Road 5A, Dhanmondi, Dhaka, Bangladesh, Tel./Fax 880 2 861-170.

Paneer Sehvam and Arun Mudgal, UNDP-World Bank Regional Water and Sanitation Group, 53, Lodi Estate, Post Office Box 416, New Delhi 110 003. India, Tel. 91-11 469 0488/9, Fax 91-11 462 8250.

2.5 Wastewater treatment using lagoons

Technical description

Lagoons play an important role as natural ecological wastewater treatment systems to reduce nutrient loading to water courses. The self-purification function of natural lagoons provide an opportunity for wastewater treatment prior to discharge or reuse. This method is especially suitable for tropical areas where there is a year round growing season and high incidence of solar irradiation. In this treatment method, wastes are degraded by various microbiological populations and pathogens can be effectively removed by aeration or exposure to sunlight. Lagoons are easy and inexpensive to construct and operate. Knowledge of this technology is quite advanced and information is readily available on the design of different types of lagoon systems. Lagoon systems are usually classified into four types: anaerobic, facultative, maturation and aerated lagoons. Each of these types is briefly described below, and more detail can be found in Yang and Wang (1990):

· Anaerobic lagoons are usually used for treatment of distillery and industrial wastes; for example, for the treatment of distillery wastewater in India.

· Facultative lagoons are usually used for removing toxic wastes. They utilise a relationship between bacteria and algae, and a balance between aerobic and anaerobic conditions to promote uptake of such chemicals.

· Maturation lagoons use micro algae and/or aquatic plants for wastewater treatment, especially for nitrogen removal.

· Aerated lagoons are an extended aeration, activated sludge process without sludge recycling. These systems usually require deeper stabilization ponds than the other types of lagoons with depths varying from 3 m to 5 m. This process is usually used for treating wastewater from both agricultural and industrial sources. It is also used for removal of nitrogen from chemically contaminated wastewaters.

Operation and Maintenance

This technology needs careful monitoring of flow rates and wastewater composition which can affect the various biochemical processes. Lagoons are best suited for domestic wastewater treatment, although, depending on the species composition of the floral and microbial communities, can be used for agricultural and industrial treatment. Certain species of plants can be very effective in removing heavy metals and similar contaminants from the waste stream.

Level of Involvement

This technology is typically implemented at the project level.

Costs

No data are available but costs are estimated to be relatively low for matruation or oxidation ponds. Costs for Aerated lagoons can be higher depending on the volume of wastewater to be treated.

Effectiveness of the Technology

A study carried out on Lake Biwa, Japan, by Kurata and Satouchi (1989) showed that the Nishinoko Lagoon has played an important role in removing nutrients from wastewater flowing into the lake. Lake Biwa is the largest freshwater lake in Japan, and is surrounded by many large and small lagoons. Eutrophication of the lake has occurred due to inflow of both domestic wastewater and runoff from cultivated areas in the lake watershed. The self-purification phenomenon within these lagoons has provided a means for wastewater treatment and treatment of runoff from cultivated fields which has reduced the level of enrichment within the lake.

In contrast to the use of lagoons for primary treatment of wastewater, maturation lagoons are considered as a tertiary treatment process and are commonly used after a series of other ponds. Maturation lagoons are fully aerobic and are usually used for microorganism removal. The performance of the ponds, however, depends upon pond hydraulic behaviour, pond depth, solar radiation, coliform decay per unit of solar radiation, and the light extinction coefficient. These factors have to be considered while considering the use of a maturation lagoon system for wastewater treatment (Yang and Wang, 1990).

Suitability

This technology is suitable in areas where natural lagoons exist near large waterbodies, or in areas where artificial ponds can be constructed.

Advantages

Lagoons can protect the main freshwater body by retaining pollutants. Disadvantages

There is a risk of exacerbating water pollution problems if the lagoons are not properly controlled, especially if natural lagoons are used. Further, the additional pollutants loadings arising from the input of wastewaters reduces the assimilative capacity of natural lagoons and their ability to buffer the larger waterbody from stormwater pollutant loads.

Cultural Aspects

There are no known problems associated with the use of this technology.

Further Development of the Technology

Further research, through pilot projects, is needed to fully understand the consequences of using natural lagoon systems for wastewater treatment. The use of artificial lagoons, howver, is a well-understood, conventional wastewater treatment technology.

Information Sources

Contacts

Environment and Sanitation Information Center (ENSIC), Asian Institute of Technology, Post Office Box 4, Klong Luang, Pathumthani, Bangkok, Thailand, Tel. 66 2 516 0110, Fax 66 2 516 2126, E-mail: ensic@ait.ac.th.

Bibliography

Kurata, A. and M. Satouchi 1989. Function of a Lagoon in Nutrient Removal in Lake Biwa, Japan, In: Ecological Engineering: An Introduction to Ecotechnology, W.J. Mitsch and S.E. Jorgensen (eds), John Willey and Sons, New York.

Yang, P.Y. and M.L. Wang 1990. Biotechnology Applications in Wastewater Treatment, Environment and Sanitation Information Centre Paper No. 29, AIT, Bangkok, Thailand.

2.6 Other technologies of wastewater treatment and reuse

Technology Description

In Southeast Asian countries, including Hold Kong, urban and surrounding areas are the centres of rapid expansion. The resources required to manage municipal, industrial and agro-industrial wastes are very often severely strained. Thus, economic growth is often accompanied by ecological damage, as industries generate considerable amounts of both solid and liquid waste products. Waste management is therefore an urgent environmental consideration. Conventional freshwater augmentation technologies involve different wastewater treatment processes such as preliminary or primary treatment; secondary treatment; and tertiary or advanced treatment techniques. As with any other environmental problem, new methodologies for improved waste handling and treatment rely on advancements in related sciences and technologies. In recent years, biological treatment of wastes has developed rapidly because of breakthroughs in biotechnology. Biologically-based technologies, therefore, are becoming an area of increasing importance as a mean of water pollution abatement and environmental rehabilitation. Nevertheless, with both conventional and bio technological wastewater treatment techniques, waste materials, when properly managed and treated, should not cause any appreciable environmental damage (Whitton and Wong, 1994).

Preliminary Treatment. Preliminary treatment is basically screening of settleable organic and inorganic solids by sedimentation and removal of materials. Approximately, about 25% to 50% of the incoming BOD5, 50% to 70% of the suspended solids, and 65% of the oil and grease is removed during the preliminary or primary treatment process. This process largely reduces the volume to be treated through secondary and advanced treatment processes, and, for some purposes such as irrigation of orchards and vineyards, may be considered sufficient treatment for reuse, depending upon the local acceptance. A bar screen made of long, narrow, metal bars spaced at 25 mm is used for preliminary treatment. The primary treatment process consists of grit removal. Basically two types - horizontal flow and aerated types - of grit removal techniques are used. Primary settling tanks are then used to remove the readily settleable solids prior to further treatment. The treatment process involves chemical treatment and flocculation, and passage through second and third stage settling tanks. A study by Chen (1993) to evaluate the effectiveness of primary treatment of municipal wastewater before discharge into the ocean indicated that the removal of suspended solids was always less than 50% while COD and BOD5 removals were in the range of 23% to 41% and 15% to 27%, respectively.

Secondary Treatment. The main purpose of secondary treatment is to remove non-settleable solids remaining in the wastewater stream after the preliminary and primary treatment process. Efficiency is estimated at about 85% removal of BOD5. This technique involves biochemical processes for the oxidative or reductive degradation of biodegradable organic pollutants, and includes such technologies as the anaerobic and facultative ponds as well as aerated lagoons previously described.

Fish Farming or Aquaculture. Fish farming has been used extensively to assist in the treatment of wastewater. It helps to reduce the levels of suspended solids and algal growth in the wastewater, and improves the quality of the final effluent, which may be used subsequently for crop irrigation and other uses. Wastewater treatment using fish ponds is a natural process that degrades and stabilizes organic wastes, while fertilizing a fish pond with organic wastes to stimulate the growth of natural biota, especially microorganisms which serve as fish food (Edwards, 1985). Systems consist of both dry and wet variants. The dry systems utilize nightsoil or faecally contaminated surface water, applied to the pond bottoms during the dry season, for aquacultural purposes in artificial ponds. The wet systems, which remain water-filled throughout the year, use similar nutrient sources to drive fish production in enclosures within ponds and natural lakes. These kinds of systems have been widely used in several Asian countries (Edwards, 1985).

Overland Flow Systems. Overland flow systems pass wastewater across slightly sloping grasslands which provide both filtration and erosion control. The technology is similar to the conventional trickling filter technology applied in traditional secondary treatment processes. When wastewater is passed across sloping grasslands, the contaminants are retained by filtration and adsorption, and organic contaminants are decomposed under primarily aerobic conditions. Intermittent feeding from parallel lanes provides aeration to the root zones of the grasses and avoids flooding of the treatment plots. In this process, the wastewater remains in contact with open air. This results in a relatively high dissolved oxygen content at the outlet of the system and helps to aerate the effluent without the need for additional energy. The efficiency of this method largely depends on the selection of the grass species, and is further influenced by specific local soil and climatic conditions. Common grasses used in this technology are paragrass (Braciera muticia), chestnut (Eleocharis dulcis), red sprangle top (Leptochola chinensis). Studies carried out at the Asian Institute of Technology (AIT, 1992) indicate that the overland flow system designed with paragrass in main lane and with other two grasses in other parallel lanes were effective in removing 80% of the suspended solids, 47% of the BOD5, 39% of the organic nitrogen, and 19% of the total phosphorus at the loading rate of 532 m3/week. This system is more effective in removing suspended solids than dissolved solids, but the research indicates that combined pond and overland flow systems can result in an high quality effluent. Combination systems also work well where the treatment requirements are high or the land available is insufficient.

Integrated Biological Pond Systems. The feasibility of an inexpensive wastewater treatment system based upon the principles of aquatic biology was evaluated by Wu et al. (1993), and an integrated biological pond system was constructed and operated for more than 3 years to purify the wastewater from a medium-sized city in Central China. The experiment was conducted in three phases, using different treatment combinations for testing their purification efficiencies. The pond system was divided into three functional regions: an influent purification area, an effluent upgrading area, and a multi-utilization area. These functional regions were further divided into several zones and subzones, each representing a particular ecosystem component. Various kinds of aquatic macrophytes, algae, microorganisms and zooplankton were effectively cooperating in the wastewater treatment in these zones within this integrated system. The system attained high reductions of BOD5, COD, TSS, TN, TP and other pollutants. The purification efficiencies of this system were higher than those of most traditional oxidation ponds or ordinary macrophyte ponds. Mutagenic effects and numbers of bacteria and viruses declined significantly during the process of purification, and, after the wastewater flowed through the upgrading zone, the concentrations of pollutants and algae evidently decreased. However, plant harvesting did not significantly affect the levels of reductions of the main pollutants achieved, although it did significantly affect the biomass productivity of the macrophytes. The effluent from this system could be utilized in irrigation and aquaculture. Some aquatic products were harvested from this system and some biomass was utilized for food, fertilizer, fodder and related uses. Finally, the treated wastewater discharged from the system was reclaimed for various purposes.

Advanced Wastewater Treatment Systems. Some contaminants, such as inorganic substances and a sizeable portion of microbiological populations, present in the waste stream remain in the effluent after the preliminary and secondary treatment processes. Amongst others, nitrates, phosphates and ammonia radicals may still be found in high concentrations. These pollutants can be removed through advanced treatment processes using autotrophic plants to take up nutrients and selected heavy metals and organic substances, flocculation of colloidal particulate matter with chemical flocculants, and removal of synthetic organic contaminants using absorbents and the oxidizing agents. Technologies used in advanced wastewater treatment systems include filtration, carbon adsorption, microstraining, chemical phosphorus removal, and biological nitrogen removal (Shah, 1994).

Filtration can remove most of the residual suspended solids, BOD5, and bacteria from the secondary effluent using multimedia or microstrainer filters. Multimedia filters contain low density charcoal for removal of particles with large grain sizes, medium density sand for intermediate sizes, and a high density medium for the smallest grain sizes. These filters can decrease the concentration of suspended solids in activate sludge-treated effluent from 25 mg/l to approximately 10 mg/l (Shah, 1994). The carbon adsorption technique is used also to adsorb persistent organic substances onto activated carbon, the organic removal capacity of which depends on the surface area of the carbon particles within the cartridge. Microstrainers can also be used to remove residual suspended solids. These filters consist of woven steel wire or a special cloth fabric mounted on revolving drums which capture the solids still remaining in the wastewater.

Only about 20% of the phosphorus in domestic wastewater is removed during secondary treatment. With phosphorus removal techniques, phosphorus is removed through chemical precipitation of the phosphorus with aluminum sulphate (alum), ferric chloride, or calcium carbonate (lime). This process requires a reaction basin and settling tank to remove the precipitate. Likewise, nitrogen is removed either chemically or biologically. The chemical process is called ammonia stripping and the biological process is called nitrification/denitrification. In the ammonia stripping process, nitrogen is removed in two stages by first raising the pH to convert ammonium into ammonia, and then stripping the ammonia by passing large volume of air through the effluent. In the biological process, secondary effluent is further aerated to convert ammonia nitrogen to nitrate nitrogen.

Wastewater Treatment and Reuse in India

In Calcutta, India, systematic reuse of wastewater for aquaculture started in the early 1940s. The sewage-fed fish ponds were initially created on about 4 628 ha in an 8 000 ha wetland area. As of 1987, about 3 000 ha of ponds remained active. The treatment process involves screening the raw sewage prior to it entering the ponds. After twelve days, the ponds are repeatedly netted and manually agitated using split bamboo rods. The agitation enhances the oxidation and mixing of the effluent, and promotes improved water quality. The pond is stocked with fish after 25 days, and additional sewage effluent is applied to the ponds during a 3 hour period in the morning, 7 days/month at an estimated rate of 130 m3/day/ha. Even though the total fish pond area has been reduced over the period of operation, total production and yield of fish has gone up from 0.6 tones/ha in 1948 to between 4 and 9 tones/ha in 1984.
(Source: FAO, 1992; Edwards, 1985,1990; and Ghosh. 1984)

During the denitrification processes, nitrate nitrogen is converted to gaseous nitrogen by bacteria under anaerobic conditions. Using these techniques, Chen (1993) found that the addition of polyaluminium-chloride (PAC) resulted in a 70% removal of suspended solids at a PAC dosage rate of 30 mg/l. If polyelectrolytes are added (at a rate of about 1 mg/l), the dosage of PAC could be reduced to around 10 mg/l with a similar result. Air flocculation, or dissolved air flotation filtration (DAFF), followed by sedimentation, resulted in the removal of more than 80% of the suspended solids at an aeration rate of 0.5-1.0 Nl air/l. This technology is more effective for smaller solids than for larger solids in wastewater. Organic removal, with either sedimentation or combined air flocculation and sedimentation processes, removed about 15% to 40% of the COD or BOD5. The efficiency of organic removal from wastewater was increased to about 60% by utilizing chemical coagulation and sedimentation treatment.

The use of advanced treatment is only recommended where major pollutants are not removed to a sufficient extent by the secondary treatment. Usually, advanced treatment are very complicated and expensive, and its use in developing countries to produce suitable effluent for aquaculture or farm purposes is not recommended (FAO, 1993).

Reuse of Wastewater in Irrigation. In the face of growing water scarcity, reuse of marginal quality water is the best alternative available for agriculture. Marginal quality water, as defined by FAO (1992), refers to water that possesses certain characteristics (such as agricultural drainage water, municipal wastewater, and brackish water) which have the potential to cause problems when the water is used for purposes other than the intended use. Converting marginal quality water to freshwater that can be used for agricultural purposes requires less complex treatment technologies than those required to produce a multi-purpose quality water. Further, use of wastewater for agriculture mimics the traditional use of night soils for agricultural purposes that has been practised in different parts of Asia from ancient times. Sewage farming was initiated in Bombay, India, as early as 1877, and, in Delhi, from 1913 (Shuval et al., 1986). In modem times, the most intensive use of wastewater for irrigation has been made in Israel. In India, modem use of sewage effluents for irrigation is reported to be about six decades old. China's sewage irrigation systems have developed rapidly since 1958. In Laos, effluent of sewage is used directly for the irrigation of 400 to 500 ha fields.

Wastewater Recycling in China: Application of Conventional Technologies

Municipal wastewater: Wastewater treatment and reuse in China has a long history, beginning in 1956 in North China. Municipal wastewater is treated to primary and secondary standards, with secondary treatment being provided by i) conventional activated sludge processes; ii) contact stabilization processes; and iii) pare oxygen aeration processes. In some cases, natural biological treatment facilities such as oxidation ponds and sewage irrigation systems are used as secondary treatment alternatives. Presently, wastewater from cities and towns in China amounts to about 99.6 million m3 of water.

Industrial wastewater. Both activated sludge systems and fixed film systems are widely used for treating organic industrial wastewater in China. The activated sludge systems are both mixed systems and ontact-stabilization systems. The fixed film systems are mainly rotating biological contactor systems, contact aeration systems, and biological tower systems. Efficiency is good, removing both BOD5 (95%) and COD (75%), but diminishes in the case of colour (50%), with the efficiency of me combined tanks being inferior to that of separate tanks in which me aeration and settling tanks are constructed separately.

Secondary treatment plants, using technologies such as sedimentation - dissolved air floatation - activated sludge, or tertiary treatment plants. using technologies such as mechanical -activated sludge - activated carbon absorption or zonation, exist in cities like Shanghai, Nanking and Beijing.

(Source: Ku, 1982)

Reuse of irrigation drainage water provides another important source of water for agricultural purposes. Conventional irrigation methods, such as flood or spray irrigation, result in excess water being applied to agricultural fields. The runoff that results, referred to as drainage water or return flows, may be collected and reused for irrigation purposes downstream. This practice, which is widespread though not well documented, can be found in many farmer-managed irrigation systems of Nepal, India and Thailand. In western hills of Argakhachi, Nepal, five parallel canals run across the base of the hills and successively collect drainage water from the farming areas upslope for reuse downstream. In the Kailai Terhi-Gurgi irrigation system, farmers have constructed parallel drainage networks to collect drainage water in the upper portions of the system for reuse in the lower portions of the area. The exact quantities of water reused through this process are not known. In some cases, rules have been formulated for the allocation of rights to reuse drainage water. In arid zones, such as in Egypt, drainage water is collected by an extensive network of covered and open drains and reused. The quantity of drainage water collected and reused during the 1988/89 hydrological year was estimated to be 2 634 million m3. The drainage water available for reuse had a salinity content within the limit of 1.5 mS/cm (Abu-Zeid et al., 1991), but the quantity of water available was reported to be decreasing and the salinity increasing.

In Tainan, Taiwan, night soil is spread over the bottoms of ponds which are empty during the winter, with additional night soil being added at intervals, about 4 to 5 times during the growing season. Several thousand hectares of ponds exist.

TABLE 8. Cost Comparison of Various Wastewater Treatment Processes.

Rank (1=best)

Initial Cost

Operation & Maintenance Cost

Life Cycle Cost

Operability

Reliability

Land Area

Sludge Production

Power Use

Effluent Quality

1

MA

SP

MA

SP

SP

MA

SP

SP

SP

2

AS

MA

AS

AL

AL

AS

AL

MA

AL

3

OD

AL

OD

OD

OD

OD

OD

AS

MA

4

AL

AS

AL

AS

MA

AL

AS

AL

OD

5

SP

OD

SP

MA

AS

SP

MA

OD

AS

SP, Stabilization Ponds; AL, Aerated Lagoons; OD, Oxidation Ditches; AS, Conventional Activated Sludge; MA, Modified Aeration Activated Sludge or Trickling Filter Solids Contactor

Note: Flexibility and expandability are similar for all types.
Source: BMA (1990)

In Bangladesh, overhanging latrines are constructed to supplement the water and nutrient supply to fish ponds during the dry season. The ponds, constructed near housing units, may be dry for part of the year and are usually filled with floodwater during the rainy season. Fish, entering the ponds with the floodwater, grow rapidly in the nutrient-rich ponds and are harvested prior to the ponds drying out. Similar systems can be found also in West Java, Indonesia, where about 25% offish ponds of 1 000 m2 or less in areal extent have overhanging latrines associated with them.

Wet pond aquaculture systems have been used mainly in Calcutta, India, and in China, where fish cultivation in wastewater is carried out in about 670 ha of ponds in 42 cities. The yields of the wastewater-fed fish ponds were about 3 to 4 times greater, and operating costs about 50% less, that those of conventional ponds.

In the Bangkok Metropolitan Area (BMA), Thailand, the modified aeration, activated sludge wastewater treatment method was found to have lowest initial cost, while stabilization ponds had the highest.

Table 8 shows the rankings of the various treatment technologies evaluated, according to operation and maintenance costs, operational ease and flexibility, land area required, power usage, and effluent quality.

Level of Involvement

These technologies can be implemented as both private and the governmental initiatives, as in China, or as local or private industrial initiatives, as in other countries. In most developing countries, innovative approaches that would encourage the increased use of such technologies have been hampered due to the absence of concrete regulatory measures and enforcement mechanisms, and, possibly, by government control of public water supply and sanitation systems.

Cultural Acceptability

People feel uneasy about the reuse of treated wastewater. Further, there are several public health hazards associated with the reuse of wastewater, especially associated with aquaculture systems. The risks are related to the potential for exposure to public health hazards during the transportation and application of night soils, and the consumption of contaminated organisms, and to the potential for the spread of disease by encouraging the spread disease vectors, such as mosquitos. These health risks, along with other cultural barriers, make the widespread adaptation of such technologies for wastewater treatment and reuse difficult.

Wastewater Recycling in Industries: An Example from Bangkok

The Phoenix Pulp and Paper Co., the largest paper mill in the Northeast Province of Thailand, is currently using and discharging process water at a rate of about 30 000 m3/day. The mill located next to the Nam Pong River la Khon Kaen, is proposing to spend about $ 26 million to recycle its wastewater for reuse within the company compound, instead of discharging it into the nearby river and eucalyptus plantations. This proposal comes at a time when the Thai government is expected to ban effluent discharges. The company also aims to reduce its effluent to about 20 000 m3/day by using new technologies to produce pulp products. This reduction will also reduce the volume of effluent discharged to eucalyptus plantations under "Project Green", and help to control seepage damage in neighbouring rice fields, which has cost the company about $86 000 in compensation to about 100 villagers,

Source: Bangkok Post August 9,1995; Mill May Use Up Recycled Water, p.3.

Further Development of the Technologies

The further development of wastewater technologies has a high potential in many parts of Asia, especially in Thailand, India and China. With the growing demand for water in the urban sector, more and more water suitable for potable use will be diverted to urban areas, increasing the need to use waters of marginal quality in aquaculture and irrigation fanning. The adoption of wastewater treatment and reuse technologies, however, will depend on many factors. Government and planners have to develop and facilitate such mechanisms to encourage people to adopt such technologies. Motivating mechanisms include environmental concerns - it is better to use treated wastewater for economic purposes rather, than directly discharging it to waterways and decreasing the waste assimilative capacity of the water courses, economic concerns - reuse of wastewater for aquaculture and irrigation can help reduce the pressure for public investment in large (and costly) water resources development projects; and legal concerns - regulatory and economic instruments can provide direct incentives to polluters to use treated wastewater for aquaculture and farm purposes.

Government Initiatives and the Future of Municipal Effluent Reuse for Irrigation

Land application of treated wastewater is a low-energy treatment system, providing economic returns from the reclamation of wastewater, especially in areas with acute shortages of water and nutrients. Research carried out at China's Beijing Agricultural University (BEU) and the India's National Environmental Engineering Research Institute (NEERI) concluded that, compared to other conventional secondary treatment methods, land application was generally better for the removal of pollutants. Because of the potential expansion of wastewater reuse technologies, these institutes have established a monitoring network.

In India, NEERI has conducted research on the problems arising from sewage farming, crop and soil responses to different wastewater treatments, the formulation of guidelines for sewage fanning systems, and direct and indirect health effects. As part of India's VIIth Plan, a multi-locational framework is envisaged, including regional research centres linked with Technology Transfer Centres that will implement 100 new schemes for sewage and sullage utilization in selected cities and townships.

In China, BAU has been actively carrying out an investigation of the environmental impacts of sewage irrigation systems. Several methods to measure the environmental quality in the study areas have been developed. The methods include identification of the pollution concentrations in crops irrigated with treated wastewater.

(Source: RAPA, 1985).

Information Sources

Abu-Zeid, M. and S. Abdel-Dayen 1991. Variation and Trends in Agricultural Drainage Water Re-use in Egypt. Water International, 4, 247-253.

Bangkok Metropolitan Administration 1990. Pre-feasibility Study on Private Wastewater Treatment for BMA, Office of the Prime Minister, Thailand.

Edwards, P. 1985. Aquaculture: A Component of Low Cost sanitation Technology. World Bank Technical Paper No. 36, Integrated Resource Recovery, The World Bank, Washington DC.

FAO (Food and Agriculture Organization of the United Nations) 1992. Wastewater Treatment and Use in Agriculture, FAO Irrigation and Drainage Paper 47, FAO, Rome.

FAO (Food and Agriculture Organization of the United Nations) 1993. Integrated Rural Water Management. FAO Irrigation and Drainage Paper, FAO, Rome.

Ghosh, G. and P.N. Phadtare 1990. Environmental Effects of the Groundwater Resources of the Multiaquifer system of North Gujarat Area, India. In: Proceedings of International Conference on Groundwater Management, AIT, Bangkok.

Ku, H. 1982. The Status and Trend of Water Pollution Control Technology in China. Water International, 7, 78-82.

Regional Office for the Asia and Pacific (RAPA)/FAO 1985. Organic recycling in Asia and Pacific, RAPA Bulletin, 2/85, Bangkok.

Shah, K.L. 1994. An Overview of Physical, Biological, and Chemical Processes for Wastewater Treatment, In: Process Engineering for Pollution Control and Waste Minimization, D.L. Wise et al. (Eds), Marcel Dekker, Inc. New York.

Shuval, H.I. et al. 1986. Integrated Resource Recovery: Wastewater Irrigation in Developing Countries, World Bank Technical Paper No. 51, The World Bank, Washington DC.