Minnesota/Wisconsin Engineering Notes

November 2001 Low Cost Parlors For Wisconsin and Minnesota Dairy Producers Siting Animal Production Operations Sizing Silage Piles New Agricultural and Alternative Energy Web Page Use Winter to Make Your Farm Safer Increasing pH by Low Level Aeration to Remove Soluble Phosphorus in Liquid Swine Manure Farmland Tile Drainage Design Workshop New Agricultural Drainage Publications Available Two New Farm Business Management Publications Available New University of Minnesota Department Head Resources Calendar

Increasing pH by Low Level Aeration to Remove Soluble Phosphorus in Liquid Swine Manure Jun Zhu, Minnesota Extension Engineer The use of advanced techniques such as chemical precipitation and biological processes to remove phosphorus (P) from wastewater is not a new topic. A great deal of work has been done to study P removal techniques to treat municipal and industrial wastewater. But, the techniques used are complex and costly, and thus may not be suitable for swine producers. In recent years, researchers have attempted to modify treatment techniques in an attempt to fit into a farm-scale waste management plan. However, limited success has been reported because the modified techniques are still expensive and require high maintenance. These modified treatment techniques were developed to produce effluent with low soluble P concentration so the water can be used as drinking water for animals without causing health problems. However, under most circumstances, the treated liquid will be applied to cropland as fertilizer or soil amendment. The criterion for producing livestock drinking water is stringent and entails significant capital and operational expenses. As long as the soluble P concentration in the receiving land can be balanced to avoid significant runoff, less expensive, low-level treatment may suffice to reduce the excess soluble P in swine manure before discharge. To date, little information is available to develop economically viable methods to remove soluble P from swine manure that are specially tailored to the needs and financial capabilities of swine producers. The mechanism behind the removal of soluble P is pH dependent. As pH increases from 7 to 12, these compounds become more stable and will not be hydrolyzed to release P into the solution. Therefore, chemical compounds such as Ca(OH)2 and Fe2(SO4)3 are often added in industrial wastewater treatment facilities to form phosphate precipitates such as Ca5(PO4)3OH, and FePO4. This reduces the P concentrations in the treated waste. However, when this process is used for treating swine manure, it is expensive and may cause secondary environmental pollution by raising the concentrations of chlorides and sulfates in the soil that receives the treated sludge. Since swine manure already contains a certain amount of Ca and Fe, under appropriate pH conditions, these chemicals may react with soluble P to form insoluble compounds. So, removal of P by using existing Ca and Fe in the manure without additional chemicals is of economic and environmental interest and deserves further research. Raising manure pH can be achieved chemically, but can also be done by aerating a manure slurry. In settled slurry, ammonia in solution is neutralized by dissolved CO2 to form ammonium bicarbonate that keeps the pH about neutral. Passing an aerating gas mixture through the slurry purges CO2 out of solution and causes pH to rise. Based on a laboratory-scale experiment conducted at the University of Minnesota Southern Research and Outreach Center at Waseca, we found that the manure pH can be raised by low-level aeration at an aeration rate of 0.016 ft³/ft³ manure/minute. For both intermittent (on and off every two hours) and continuous aeration treatments, the manure pH increased by about 1 unit (from 6.5 to 7.5) within the first day of operation. This increase in pH was accompanied by a 75% reduction in soluble P concentration in the liquid manure. In this study, we also found that continuous aeration did not show an advantage over intermittent aeration in removing soluble P. So, using intermittent aeration can reduce energy consumption by 50%, as compared to continuous aeration. The information obtained from this study has indicated that it is possible to effectively reduce soluble P concentration in liquid swine manure at a potentially reduced cost. With this information, further research can be directed to the development of a simple aeration system that can be applied to the current manure storage to provide minimum aeration for a short period of time for P removal. The liquid portion of the treated manure that contains low soluble P can then be spread immediately after the treatment, while the solid portion can be processed separately. In this way, the environmental concerns about P pollution due to swine production might be significantly reduced.

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