Land Treatment Systems
In the future the main threat to the normal development of the economy and life will not be water deficiency but the conversion of rivers, lakes and other water supply sources into sewers. This requires cessation of wastewater discharge into streams and proper treatment of wastewater. Conventional domestic wastewater treatment system need huge investment of capital and thus becomes unviable economically due to ever increasing discharge of wastewater. The situation therefore demands an economical and technically acceptable alternative treatment technology.
Over the last few years there has been renewed concern over the application of various types of effluents to land. The soil is considered as an effective filter system which renovates the quality of wastewater. This approach combines both treatment and reuse of the effluent and operates according to the ‘zero discharge’ concept.
Table of Contents
Land Treatment as an alternative method
The land treatment is one alternative where wastewater is disposed onto land. According to Crites, land treatment refers to the application of partially treated wastewater to the land that is designed, constructed and operated to treat wastewater through the use of crops, irrigation methods, ground and surface water monitoring to conform to specific water quality limits. It involves the controlled application of wastewater to the land at rates compatible with the natural physical, chemical and biological processes that occur on and in the soil. Point-source control, wastewater transmission, pretreatment, distribution water techniques, plant selection, and use of regenerated water determine the efficiency of the land treatment system (LTS). In India, LTS popularly known as sewage farms, are very common wherever sufficient land suitable for the purpose is available. These systems can often be the most cost-effective option in terms of both construction and operation and are therefore, frequently being used in small communities and rural areas.
The use of domestic wastewater emanating from these communities on fast growing plant species can be an effective way of wastewater treatment as well as a source of water and nutrients for growing plants. The application of domestic wastewater for irrigation to food crops generally fulfills their nutrient requirement but in other hand make them more vulnerable to the attack of insects and pathogens. Hence, the irrigation of trees with this wastewater is considered as more economical and eco-friendly method of fertilization. The species like Poplar and Salix have longer growing seasons and deeper, longer lasting root systems than annual crops, which enables them to have a better utilization of the nutrients from wastewater. Secondly, these plant species possess high rate of evapotranspiration which further enhances the LTS treatment efficiency. The net effect is the beneficial system that provide effective waste remediation and recycling of water, nutrients and carbon via biomass production (Bastian and Ryan, 1986).
Land treatment techniques
The technical design of the land treatment system mainly depends on the mode of wastewater application, and characteristics of wastewater and on-site soil profile. The parameters that should be given utmost consideration during land application are dissolved salts, suspended solids, nutrients like nitrogen and phosphorus, organic matter, cations like sodium and magnesium, and toxic substances. The important site conditions include the depth of the soil mantle, depth of ground water table, slope and permeability. The land based treatment of wastewater based on how it is applied over land can be classified as:
1. Slow rate (SR) method
2. Rapid infiltration (RI)
3. Overland Flow (OF)
The Slow Rate (SR) method
wastewater is applied on a land with vegetation at a slow rate to avoid wastewater runoff and it is treated both by plants and microorganisms present in the soil. The applied wastewater is either consumed through evapotranspiration or percolated vertically and horizontally through the soil system. Any surface runoff is collected and reapplied to the system. It submerges ultimately into the groundwater. This technology incorporates wastewater treatment, water reuse, crop utilization of nutrients and wastewater disposal. The wastewater can be applied to vegetated land by means of various techniques, including sprinkling methods or surface techniques such as border and furrow irrigation. However, the application is intermittent (every 4 to 10 days) to maintain aerobic conditions in the soil profile. Treatment occurs as the wastewater percolates through the soil profile. SR systems are considered to have the highest treatment potential of all natural treatment systems.
Rapid infiltration (RI)
Rapid Infiltration is the most intensive of all land treatment methods. Relatively high hydraulic and organic loadings are applied intermittently to shallow infiltration or spreading basins (see figure). The RI process uses the soil matrix for physical, chemical, and biological treatment and it is devoid of any form of vegetation. Physical straining and filtering occur at the soil surface and within the soil matrix. Chemical precipitation, ion exchange and adsorption occur as the water percolates through the soil. Biological oxidation, assimilation and reduction occur within the top few feet of the soil. The treated wastewater finally submerges into the water table.
The RI system is designed to meet several performance objectives including the following:
(a) Recharge of streams by interception of groundwater;
(b) Recovery of water by wells or underdrains, with subsequent reuse or discharge;
(c) Groundwater recharge;
(d) Temporary storage of renovated water in the local aquifer
Overland Flow
Overland flow (OF) method has intrinsic potential for advanced wastewater treatment and offer many advantages such as low construction and operation costs, easy operation, low detention time, and low-energy requirement. Wastewater is applied intermittently to the top portion of a sloping land grown over with grass and flows down the terrace to a runoff collection channel at the bottom of the slope (fig. 2b). The wastewater then flows into collection pipes located at the end of the slope. Application techniques include high-pressure sprinklers, low-pressure sprays, or surface methods. OF is normally used with relatively impermeable surface soils, since, in contrast to SR and RI systems, infiltration through the soil is limited. OF systems can be designed for secondary treatment, advanced secondary treatment or nutrient removal, depending on user requirements.
Environmental risks of LTS
Concerns for human health and the environment are the most important constraints in the wastewater reuse. Important drawbacks associated with the land application of wastewater include nutrient release and spread of pathogens. Nutrient releases to the environment, including N and P transport in surface waters, nitrate leaching to groundwater, ammonia emissions, and greenhouse gases, are of concern due to their potential for adverse effects on the quality of waterways, biodiversity, public health and the climate. Efficient planning, design, operation and management of LTS is therefore required to reduce nutrient losses to the environment.
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References
Bastian, R. K., J. A. Ryan (1986), Design and management of successful land application systems. In Proceedings. Utilization, treatment, and disposal of waste on land. Soil Science Society of America, Madison, Wisconsin. pp. 217-234.
Crites, R., Tchobanoglous, G. (1998), Small and Decentralized Wastewater Management Systems. McGraw Hill, New York.
EPA (1976), Land Treatment of Municipal Wastewater Effluents, Case Histories: Environmental Protection Agency, Technology Transfer.
