Origin, Nature, and Treatment of Effluents From Dairy and Meat Processing Factories and the Effects of Their Irrigation on the Quality of Agricultural Soils

Dairy and meat processing industries use large volumes of water to maintain clean, hygienic conditions and are, therefore, major generators of wastewaters. These contain a high organic load, significant quantities of cleaning and sanitizing compounds, relatively high N, P, and Na contents, and show large fluctuations in flow rate and composition on an hourly daily and weekly basis. Before disposal, wastewater undergoes primary and usually secondary treatment. Anaerobic and aerobic digestion methods can be used in secondary treatment, and these can involve extensive (e.g., lagoons) or more costly but more compact alternatives (e.g., suspended biomass and immobilized cell reactors). Treated effluents can be (a) discharged to surface waters, (b) discharged to sewers, or (c) most commonly, irrigated onto land. Indeed, effluent can constitute a cheap source of water and nutrients for pasture/crop production on land surrounding the factory. Annual wastewater application rates are normally governed by the maximum allowable N loading rate and use of spray irrigation systems is favored. Irrigation is not without potential environmental problems such as (a) groundwater contamination, particularly with NO3-; (b) excessive accumulation of Na and an attendant diminution of soil structure; (c) accumulation of soil P; (d) nuisance odors and aerosol drift; and (e) surface runoff to waterways. As a result, an ongoing monitoring program including regular soil and groundwater analysis is typically required. Despite this, a review of literature revealed effluent irrigation has many positive effects on soil quality, including (a) a liming effect, (b) a modest increase in organic matter content, (c) a reduction or elimination of the need to apply N and P fertilizers, (d) improved aggregate stability when the soil is left undisturbed, and (e) an increase in the size and activity of the soil microbial community. Management of irrigation systems should aim to minimize any detrimental environmental effects while maximizing the positive effects on soil quality and plant production.