Dewatering Dairy Manure Using
Polymer and Belt Press Technology

L. Alice McKenney
Project Administrator, Tuscarawas Soil and Water Conservation District
New Philadelphia, Ohio

The East Branch Sugar Creek Dairy Waste Separation and Treatment Demonstration Project has been investigating the potential of polymer and belt press dewatering technology, now used in municipal and industrial waste water treatment, as a method of increasing the bulk density and nutrient content of dairy manure. The goal was to provide an economically attractive alternative to existing manure transportation and handling methods, thus encouraging the transportation of nutrients from nutrient rich to nutrient deficient cropland areas.

Located in Tuscarawas County, Ohio, the 29 square mile East Branch Sugar Creek Watershed is home to 16 dairy operations with an estimated 7,800 animal units. The Tuscarawas Soil and Water Conservation District in 1994 applied for and received a grant from the Ohio Environmental Protection Agency under provisions of Section 319(h) of the Clean Water Act as amended in 1987. Project co-sponsors include Or-Tec, Inc., an Ohio-based manufacturer of wastewater treatment equipment, and Calgon Corporation, a manufacturer of blended and emulsion polymers.

This project was not planned as a research project, but rather as an on-farm demonstration of an already proven municipal/industrial wastewater technology. Project sponsors envisioned a rapid transfer of this technology to agricultural use in the dairy industry, but this has proved not to be the case.

Inherent in the design of mechanical manure separators such as the slant screen, rotating screen, or screw press type, only particles larger than the screen mesh are captured. Most reported removal rates for total solids are only 20% to 35%, with removal rates for nutrients even lower (Barker, 1993; Moore, 1989). Higher removal rates for both total solids and nutrients can be achieved by clumping the fine and colloidal particles together, through the processes of coagulation and flocculation, prior to mechanical removal (Calgon Corporation, 1994).

The suspended colloidal solids in manure carry a negative or anionic surface charge, which disperses the particles and keeps them in suspension. This surface charge can be eliminated by introducing the correct dose of a coagulant, a product with a strong positive or cationic charge. Once this surface charge has been eliminated, the particles will be able to come together and form larger particles. It is sometimes helpful to add a second product, which has the ability to flocculate these particles, sweeping them together into a larger, denser floc, which is more easily removed. A high molecular weight polymer is ideal for this purpose (Calgon Corporation, 1994). Once the dense floc has formed, it can be removed by mechanical means, such as the belt press.

This technology was first demonstrated in October 1995 at three dairy farms in the watershed. Each farm represented a different method of manure storage and handling:

1. Combined scraped manure and milkhouse waste in an under-barn concrete vault,
2. A flush system, and
3. An earthen impoundment with scraped manure only.

A standard belt press demonstration unit with a modified sludge feed and chemical dosing unit was operated. The primary coagulant used contained aluminum sulfate as its main active ingredient, one of the most common EPA approved coagulants used today in both drinking water and waste water treatment.

Problems with poor coagulation were encountered with the flushed manure. Scraped manure with about 11% to 12 % total solids repeatedly clogged the positive displacement progressive cavity pump. The best results were obtained at the farm with dairy waste consisting of combined scraped manure and milkhouse wastes, averaging 8.3% total solids. This slurry was dewatered to an average of 22.8% total solids with both phosphorus and nitrogen concentrated in the solids. At this farm the pump also occasionally became clogged with fibrous materials, interrupting the process.

At the end of the first year of the project, the belt press company agreed to explore other pumps and the chemical company agreed to evaluate other possible polymers, not containing aluminum. Dairy operators in the project advisory group did not want to land apply additional aluminum to fields where forage crops are grown for dairy cows. Their concern is supported by studies reported in Nutrient Requirements of Dairy Cattle (National Research Council, 1988). The group also decided to continue testing exclusively at the farm where the best results had been obtained.

During the summer of 1996, the problem of clogging in the pump and lines was solved by adding an electric agricultural chopper pump to lift the manure from the pit to a stainless steel equalizing tank with an overflow back to the pit. This preconditioned the slurry in two ways: (1) the solids in the manure were reduced to smaller more uniform pieces and (2) the suspension of the solids was improved allowing for more consistent inlet solids content to the belt press process.

The chemical company conducted extensive lab testing of existing water and waste water treatment chemicals in their product line with samples of dairy manure from the selected farm. When this testing failed to identify an effective coagulant, they formulated two promising non-aluminum bearing products; DW-150 a polymer described as an "aqueous solution of a modified cationic polyacrylamide" and DW-200, a similar emulsion type polymer. On site testing of the prototype agricultural belt press continued in spring of 1997 using the emulsion type DW-200, chosen because a simpler method for dilution and dosing was available and it required less dilution water.

After pre-conditioning, as described above, the slurry was pumped out of the stainless steel equalizing tank, through a two-inch flexible hose, using a variable speed positive displacement progressive cavity pump. Injection ports before and after the pump allowed for split injection of the polymer solution. This provided mixing of the manure slurry with the polymer solution both from turbulence in the line and inside the pump itself. After the pump, further gentle mixing took place inside the approximately eight feet of serpentine two-inch clear plastic pipe which fed into the bottom of the final flocculation tank. The flocculation tank consisted of a 30-inch vertical clear plastic cylinder, six inches in diameter and containing a variable speed mixing rod with seven propeller-like paddles.

The flocculated manure contained large clumps of solids surrounded by visibly clearer water. It flowed down a feed chute and was distributed gently over the gravity drainage section of the dewatering belt. The slowly moving belt could be adjusted to travel at speeds of 8-12 feet per minute. The belt itself was an industry standard belt with a low permeability rating, typically used for fibrous materials. Most of the liquid removed from the manure drained through this section of the belt, flowing into the drainage tray. The solids remaining on the belt then moved under a single squeeze roller where additional liquid was released. (See Figure 1.)

In fourteen days of test operation in summer 1997, dairy waste ranging from 5.4% to 8.3% total solids was dewatered to 20% to 23% total solids. Removal rates varied as testing conditions varied from day to day. For example the dilution rate for the emulsion polymer was varied, injection points were changed, etc. Removal rates for total solids were consistently in the 71% to 78% range. Most phosphorus (P) removal rates were between 69% and 75%. TKN Nitrogen removal rates were between 31% and 41%. Potassium removal was not significant and varied widely.

Polymer usage rates were in the range of 2.5 to 4.5 gallons per hour of operation, processing manure ranging between 5.4% and 8.3% total solids, at flow rates varying from 140 to 720 gallons per hour. Polymer usage was calculated in terms of pounds of polymer used per ton of dry solids produced. During these trials, the weight of polymer used ranged between 480 to 580 pounds of polymer per ton of dry solids produced. At a cost of $2.20 per pound, the chemical cost would be $6 to $7 per day per cow, based on a typical 1400-pound dairy cow producing manure containing 14.6 pounds of total solids per day (Midwest Plan Service, 1993).

This project accomplished the goal of dewatering dairy manure and concentrating nitrogen and phosphorus into the separated solids. While results varied slightly throughout the testing period, on the average the same quantity of TKN nitrogen and P phosphorus found in the unprocessed manure was concentrated into separated solids weighing only 42% to 50% as much. No assumptions about the efficacy of this process for other animal manures have been made, nor should any be inferred, as the chemical and physical properties of manures vary.

Although demonstrating this as an economical alternative to current manure handling practices was not achieved, agricultural application of polymer and belt press technology should not be ruled out based on this project alone as this was not an exhaustive investigation. There are many variables, such as using belts with different weaves and permeability, different roller configurations, etc. However, while research into these parameters may improve the efficiency of polymer usage, it is unlikely to bring usage rates of the DW-200 in line with polymer usage rates in processing other organic waste streams of similar total solids content.

Typical polymer usage rates in wastewater applications are in the range of 8 to 20 pounds per ton of dry solids produced. Usage rates may range as high as 50 pounds per ton. At 480 to 580 pounds per ton, polymer usage in these trials was exponentially higher. This suggests that although the DW-200 emulsion polymer used was effective in separating the solids for dewatering, and produced reasonably good removal rates for total solids, phosphorus and nitrogen, it is not a practical choice for agricultural application of this technology to dairy manure.

In the evaluation of this project other practical issues have been identified which are engineering problems, and could most likely be resolved. This project did not answer the fundamental question which remains for further study, namely: What is it about the physical and chemical properties of dairy manure that differ so markedly from the biosolids in municipal/industrial waste waters, that could cause the polymer demand for dairy manure to be exponentially higher than that for municipal/ industrial wastewater solids?

1. Dairy manure can be dewatered using polymer and belt press technology.
2. Solids dewatered by this method contain significantly more of the phosphorus and nitrogen than solids dewatered by mechanical separation alone.
3. Aluminum sulfate as the coagulant was not an acceptable choice to dairy operators.
4. The ratio of the pounds of DW-200 emulsion polymer used to one ton of solids produced was significantly higher than usage rates of emulsion polymers in municipal/industrial waste water applications with waste streams of similar total solids content.
5. The usage rate and cost of the DW-200 emulsion polymer make it prohibitively expensive for use in processing dairy manure.

1. Basic research into the physical and chemical properties of dairy manure.
2. Basic research in the development of polymers specifically for dewatering dairy manure.

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