The application of wastewater to land is a low-capital and low-operating cost method for the treatment of food processing wastes, provided sufficient land with suitable characteristics is available. This method has been the most highly used 'technology' for wastewater treatment for a long time. Obviously, it cannot be very effective and recent legislation tends to limit the possibility of spreading food wastewaters on land. However, this system still has its importance in wastewater treatment.
The ultimate disposal of wastewater applied to land is by one of the following methods:
• percolation to groundwater,
• overland runoff to surface streams,
• evaporation and evapotranspiration to the atmosphere.
The following methods are used for land application:
• surface ponding,
• groundwater recharge by injection wells,
• subsurface percolation.
Generally, irrigation methods are used the most frequently. Irrigation processes can be further divided into four subcategories according to the rates of application and ultimate disposal of liquid. These subcategories are overland flow, irrigation, high-rate irrigation and infiltration-percolation.
With respect to organic carbon removal, these systems have been shown to achieve pollutant removal efficiencies of approximately 98 and 84% for the infiltration and overland flow systems, respectively. Nitrogen removal is found to be slightly more effective with the infiltration-type land application systems when compared with the overland flow application. However, the infiltration type of application has been shown to be quite effective for phosphorous and grease removal and thus offers a definite advantage over the overland flow application if phosphorous and grease removal are of prime importance.
According to Carawan et al. (1979), irrigation is a treatment process that consists of a number of steps:
1 Aerobic bacterial degradation of the deposited suspended materials, evaporation of water and concentration of soluble salts.
2 Filtration of small particles through the soil cover, and biological degradation of entrapped organics in the soil by aerobic and anaerobic bacteria.
3 Adsorption of organics on soil particles and uptake of nitrogen and phosphorus by plants and soil microorganisms.
4 Uptake of liquid wastes and transpiration by plants.
There are both hydraulic and organic loading constraints on the use of this method for the ultimate disposal of effluent. If the maximum recommended hydraulic loadings are exceeded, the surface runoff would increase. Should the specified organic loadings be exceeded, anaerobic conditions could develop with resulting decrease in BOD5 removal and the development of odour problems. Applied loadings of organic suspended solids average approximately 8 g/m2 but loadings up to 22 g/m2 have been applied successfully. A resting period between applications is important to ensure survival of the aerobic bacteria. The spray field is usually laid out in sections such that resting periods of 4-10 days can be achieved.
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