(332d) Phosphorus Recovery from Liquid Dairy Manure By Electrocoagulation

Authors: 
Hu, B., University of Minnesota
Zhang, X., University of Minnesota
Lin, H., University of Minnesota
Rajendran, A., University of Minnesota
Reis, C., University of Minnesota
Liu, W., University of Minnesota

Dairy manure, containing high levels of volatile solids, N, P and K that are necessary nutrients for plant growth, is conventionally used as orgnic fertilizer. Removing liquid from the manure solids, by natural sedimentation or mechanical separation, can enrich volatile solids and nutrient content, and therefore decreases its transportation cost for land applications. However, the separated liquid phase still contains soluble and colloidal forms of phosphorus and direct discharge of the manure liquid may cause eutrophication. Collecting the remaining phosphorus from the liquid will increase the fertilizer value of dairy manure and eliminate the issues associated with nutrient contamination.  Proper methods should be developed to recover the remaining phosphorus which is difficult to be physically separated from the manure liquids. Electrocoagulation (EC) is a method suitable to decrease pollutants that are difficult to be removed by filtration or centrifugation. So in this work, the possibility of enriching P from the liquid manure was evaluated.  First, four electrode materials, including 304 stainless steel, aluminum, low carbon steel and grey cast iron, were compared for their effects in P removal from liquid dairy manure (filtrate of 295-μm mesh screen). Before electrocoagulation, about 80% of solids (on dry basis) in the liquid dairy manure were smaller than 45 μm. After EC treatment, manure particles of small sizes were coagulated and a natural sedimentation for 20 min successfully separated most of the P-bearing solids, evidenced by the increased proportion of particles larger than 250 μm. Although aluminum showed the best P removal performance, aluminum anode leaded to the formation of a thick oxidation film, which stopped the further release of coagulants. Among the iron-based electrodes, the best P removal efficiency of 90.2% was achieved in 100 min by stainless. However, its iron content is only 53.5-74.5% and the other components like chromium and nickel will bring further environmental pollution. Low carbon steel, removing 87.1% total phosphorus within 100 min, had a better P removal efficiency than cast iron. This material also had high iron content and low electrical resistivity, and therefore was used as the electrode material for further exploration. Second, experiments were conducted to optimize the EC process for P removal and to monitor several relevant property changes  of sludge and supernatant, such as pH, solid profile and content, iron content, et al. The operating parameters included current (0.2-1.4 A), initial pH (5-9), aeration (0-120 min treatment), agitation speed (0-300 rpm) and anaerobic digestion (manure was digested for different duration before experiments). It was found that alkaline adjustment of the liquid manure significantly improved phosphorus removal. Higher current produced coagulants at a higher rate at anode but consumed more electrical power, and thus a cost analysis was carried out to clarify the relationship between the energy consumption and the P removal in various operating conditions. Aeration treatment increased pH and stripped out carbon dioxide and hydrogen sulfide from liquid manure, which was found to have accelerated the P removal in the following EC treatment. Aeration for 60 min at a rate of 0.8 L/min before electrocoagulation process had the best P removal performance.  Agitation could increase the mass transfer of coagulants in the medium but meanwhile the coagulated particles would be broken down when agitation speed was too high. As a result, the agitation rate of 75 rpm was found to have the best P removal rate. Anaerobic digestion (AD) could reduce the organic matter in manure and thus reduce the solid content. It is assumed that the energy provided during EC process has an effect on both P removal and solid coagulation. More energy needs to be contributed to P removal due to the low solid content in anaerobic digested manure. 97.26% of total phosphorus was removed within 70 min when using the manure after anaerobic digestion for 10 days. Finally, empirical models were established to predict the treatment performance, reaction time and energy consumption at given operating conditions. The EC method developed in this study successfully achieved the goal of expediting manure solid/liquid separation and P recovery. The produced EC sludge will be evaluated for fertilizer use.