Options for dewatering manure

Different technologies are available to reduce moisture content of wet manure and help create valueadded products.

ONE SOLUTION to nutrient overloading from manure is creating value-added products that can be economically shipped outside of an overburdened watershed. Value-added manure products can be produced by composting, chemical stabilization, dehydration and pelletizing. For all of these technologies, the manure must be reasonably dry at the start. With the exception of litter from broiler and turkey farms, most manure comes out of the barn wet. Typically, moisture contents of raw manure range from 85 to 98 percent, depending on how much bedding is used and how the manure is collected (e.g. flushed or scraped).

Moisture content can be reduced by adding dry amendments, as is often practiced in composting. However, it is often preferable to reduce moisture content simply by removing water - dewatering. Unlike adding dry amendments, dewatering minimizes the amount of material that needs to be handled and processed. Several technologies are used for reducing the moisture content of wet manure, including gravity separation (e.g. settling), natural drying, mechanical separation and drying with heat (i.e. dehydration). Each technology has its target applications and limitations in removing water.

GRAVITY SEPARATION

Gravity separation methods, such as settling basins, are often the first step in dewatering liquid manure. In a settling basin, the velocity of liquid manure flow decreases, causing the solids to settle by gravity to the bottom of the basin. The mostly-liquid portion above drains away through a pipe or over a weir. Periodically, the solids are harvested from the basin. Various types of settling devices are used, from concrete vessels to earthen ponds. They are used for liquid manures, primarily to capture solids from liquid streams rather than lower the moisture content of solids. Gravity separation is only partially effective. A large amount of solids escapes with the liquid, and the harvested solids still have a very high moisture content. However, it is now practical to dry the solids by other means.

NATURAL DRYING

Under the right climatic conditions, evaporation beds, ponds, and small piles can effectively remove moisture by natural aeration. Essentially, these devices promote evaporation, maximizing the exposure of the manure to air, wind and solar radiation. Agitation is often used to enhance drying. Natural drying methods are mechanically simple and inexpensive, although they require a relatively large land area. Potentially, they can produce very dry manure. However, they are at the mercy of the weather so natural aeration techniques are only effective in dry climates or dry seasons.

MECHANICAL SEPARATORS

Because of the limitations of gravity separation and natural aeration, many farms use mechanical dewatering equipment. Mechanical dewatering techniques are used both to reduce the solids load on liquid manure handling systems and to recover solids at a lower moisture content for composting or as bedding. Mechanical separators employ a combination of pumps, screens, conveyors, rollers and other devices to separate water from manure solids. Several different mechanical separators are used. Stationary screens, roller presses and screw presses are three common types. Most stationary screens are simple sloping screens. After liquid manure is pumped on the screens, the liquid drains through the holes while solids remain on the surface and eventually slough off into a pile. Roller presses also use screens (flat or circular) but add a set of rollers that squeeze out additional water from the solids before they leave the screen. Screw presses squeeze water from manure with a rotating auger or screw inside the press. The auger forces manure against a screen, pushing water out the screen holes. The squeezed solids are conveyed out of the press by the auger. Stationary screens can produce solids with a moisture content in the 80 to 90 percent range. Solids from roller and screw presses typically range from 75 to 80 percent moisture.

DEHYDRATION

Driving off moisture by heat and air flow is warranted when the manure must be very dry and natural drying isn't feasible. In a sense, composting and lime stabilization perform this using the heat generated by the process to fuel drying. Most manure drying systems use fossil fuel for heat. Dehydration take places in vessels such as rotary drums or cyclones. Cyclone type mechanisms employing high-speed air are attracting interest because they can both evaporate moisture efficiently and reduce particle size.

Drying systems - whether natural or with fuel-heated air - concentrate the nutrients in the solids, although some nutrients, such as ammonia, are lost in the process. In contrast, gravity and mechanical separation produce two materials that must be managed liquids and solids. The lion's share of the nutrients remains with the liquid.