Lime Precipitation and Phosphorus Removal from Dairy Wastewater

Linda Benedict, Moreira, Vinicius R., Sheffield, Ron  |  7/1/2010 2:04:21 AM

Ron E. Sheffield, Vinicius R. Moreira, Brian D. LeBlanc and Troy Davis

Nutrient management and recovery on livestock farms, such as dairies, is important for two reasons. The major concern is the gradual buildup in the soil of nutrients such as nitrogen and phosphorus from manure application. When manure is applied to crops at appropriate rates for nitrogen fertilization, phosphorus is usually over-applied. Over time, this localized phosphorus surplus can lead to declining surface water quality and eutrophication – the accelerated nutrient enrichment of water bodies – from runoff. The second important concern in addressing phosphorus recovery is the anticipated worldwide decline in high-quality phosphorus reserves. At the current rate of extraction, high-quality phosphorus reserves will be depleted within a century. Phosphorus recovery can be used as a tool to address the nutrient imbalance and can help to protect water quality in the process.

One strategy to remove phosphorus from dairy wastewater in a cost-effective manner is to remove the dissolved phosphorus in wastewater and form a granular crystal that can be easily col lected and shipped off the farm. Unfortunately, this system is cost-prohibitive for most producers. Another strategy – an electro-coagulation system – has been tested and found to be effective in removing phosphorus. The resultant sludge from the system, however, is small in size and is extremely difficult to handle without releasing the phosphorus back into the wastewater. But the system that is being adapted for use in Louisiana dairies is a lime-precipitation system.

How It Works
The lime-precipitation system is designed to use hydrated or burnt lime to remove phosphorus from dairy parlor wastewater and in the process destroy pathogens. As wastewater flows into a mixing tank, a differential pH controller senses a drop in the pH and triggers a pump to add a mixture of a 30 percent hydrated lime solution to the wastewater. The lime reacts quickly with the wastewater, causing dissolved phosphorus in the water to react with the calcium and create calcium phosphate, which precipitates out of the water.

The material flows from the mixing tank into a cone-shaped clarifier, and the precipitate begins to settle. As it settles, it captures other solids and nutrients. As a result, the treated water is nearly clear and low in odor. The low-phosphorus wastewater flows from the top of the tank while the high-phosphorus solids concentrate in the cone where they can be pumped or drained, separated and removed from the farm.

Experiences with Lime Precipitation
Laboratory tests simulating the lime-precipitation system were conducted on wastewater from the LSU AgCenter Southeast Research Station located approximately 40 miles north of Lake Pontchartrain. The station houses a 200-head dairy herd in complete confinement in a sand-bedded, flushed, freestall barn. Manure from the barn is handled separately from the milking parlor in a two-stage earthen storage facility.

The addition of a hydrated lime solution to the raw, nonseparated parlor wastewater was found to be effective in reducing the concentration of dissolved and total phosphorus. Dissolved phosphorus in the parlor wastewater was approximately 20 percent of the total phosphorus concentration. Additions of 1 percent and 5 percent of the hydrated lime solution resulted in reductions of 46 percent and 78 percent in total phosphorus, respectively, with corresponding reductions of 71 percent and 63 percent in dissolved phosphorus.

The use of the lime-precipitation system to remove phosphorus from dairy wastewater also was extremely effective in reducing the potential risk from waterborne pathogens. The addition of a 5 percent hydrated lime solution (Figure 1) was found to have a 99.9999 percent reduction on fecal coliform bacteria from the raw dairy parlor wastewater and a 99.9998 percent reduction when added to the lagoon effluent. The existing lagoon system reduced fecal coliform bacteria by 99 percent. This is similar to results reported by researchers from University of North Carolina-Chapel Hill for swine lagoons in North Carolina.  

System Economics
Based on a 500-cow dairy, a 7,600-gallon clarifier (12-foot diameter and 6-foot tall with a 9-foot collection cone) would be needed. Several local steel fabricators estimate the clarifier cost to be $15,000. The effluent mixing tank, pH controller and lime slurry mixing tank can be constructed for approximately $8,500. Operating cost estimates for a 500-head, open-lot dairy is approximately 6 cents per cow per day to remove 80 percent of the phosphorus, assuming the cost of burnt lime to be $150 per ton. Reducing the removal rate to 45 percent reduces the daily cost to 2 cents per cow per day.

Current Work
The AgCenter has constructed a 350-gallon pilot-scale lime precipitation unit. To test for reliability, the system uses mixers and lime slurry pumps similar to those that would be used on any full-scale system. Tests to optimize the system operation on dairy parlor wastewater, primary lagoon effluent and flushed freestall barn wastewater will be evaluated in 2010. In addition to evaluating the pilot-scale unit, the AgCenter is developing a novel system using rice hulls for microbial nitrification – the conversion of volatile ammonia into stable nitrate. It is anticipated that nitrification will increase the nitrogen value of the wastewater and provide a more balanced nutrient supplement to crops.

Work also is being done with researchers and extension specialists in New Mexico, Texas, Colorado and Indiana to develop phosphorus removal systems appropriate for their dairies.

Ron E. Sheffield, Associate Professor, Department of Biological & Agricultural Engineering; Vinicius R. Moreira, Associate Professor, Southeast Research Station, Franklinton, La.; Brian D. LeBlanc, Associate Professor and Roy and Karen Pickren Professor of Water Resources, LSU AgCenter and Sea Grant, and Callegari Environmental Center, Baton Rouge, La.; Troy Davis, Graduate Research Assistant, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2010 issue of Louisiana Agriculture.)

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