|Sourcebook of Alternative Technologies for Freshwater Augmentation in some Asian Countries (UNEP-IETC, 1998)|
|Part B - Technology profiles|
|2. Wastewater treatment and reuse technologies|
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.
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).
This technology is suitable in areas where natural lagoons exist near large waterbodies, or in areas where artificial ponds can be constructed.
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.
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.
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: [email protected].
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.