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.

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.

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.

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.

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.

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.