Making sense of manure treatment technology options
EAST LANSING, MI. - When it comes to manure, technology holds promise for the dairy industry.
"The goal is to improve the sustainability of how we apply and how we use manure on crops and find more value in the manure," Dr. Dana Kirk, biosystems and agricultural engineer with Michigan State University's Anaerobic Digestion Research & Education Center, said during a presentation on manure treatment technology options presented by DAIReXNET.
The question is whether a dairy wants to treat its manure and, if so, what technology will work best for it. That depends on the characteristics of the manure produced, including its moisture, nutrient content and contamination, Kirk said, as well as how the farm beds and handles its manure collection and conveyance.
The farm's reason for treating manure also figures in. It may be to reduce odors or the cost of hauling by concentrating nutrients and phosphorus, to create bedding or develop a new revenue stream.
Recovering nutrients from manure
The first step, separating manure into coarse solids and liquid, reduces pipe clogging, sludge accumulation in storage, storage crusting, and makes land application easier. The material pulled off can be used as soil amendment or compost.
Coarse solids separation can be done with a slope screen or a rotary drum screen, which use wedge-wire and screens that diluted manure is pumped over to remove the coarse solids. The solids can go through rollers or screw presses to remove additional water.
Coarse solid-liquid separation removes, roughly, 15-30 percent of the phosphorus and nitrogen. The cost is relatively inexpensive.
The next step would be separating the fine solids from the liquid. Depending on the technology used, this can remove over 85 percent of organic nitrogen and 90 percent of phosphorus.
This can benefit farms with land application restrictions, Kirk noted. The concentrated material can reduce truck traffic/cost to haul and may offer a nutrient market opportunity.
The wide range of technology options available include polymer or coagulant flocculation, which use chemicals to bind particles, as well as presses involving moving discs, belt filters or plates.
Centrifuges, vibrating and incline screens can be used, as well as struvite crystallization, which produces a dry, high phosphorus content, slow-release, pelletized fertilizer.
The fine solids can be used as compost, fertilizer, soil amendments or to make phosphorus cake, while the liquid becomes effluent filtrate tea water for irrigation purposes.
Energy generation/ thermal conversion is another approach. Pyrolysis uses moderate temperatures in the absence of air to convert biomass solids into more energy-dense forms. The resulting combustible gases and liquids can be further refined to liquid fuels and biochars.
Torrefaction is a lower temperature version of pyrolysis, aimed at densifyiing the energy content of biomass solids by producing a bio-char, or charcoal.
Gasification uses high temperatures and controlled combustion with air to completely convert solid biomass into gases that can be used for heat, power and fuel.
Anaerobic digesters are a very good biological process to create natural gas from dairy manure, Kirk said. Systems include plug flows, complete mixes and covered lagoons.
For fiber drying for farm bedding and marketable products, farmers can use drum driers or belt driers, which use a series of perforated steel plates that carry solids across a horizontal flow of warm air.
Composting drums are rotating drums with conveyed, separated manure solids rotating against hot air, leading to very quick pathogen reduction and a product that can be used for fertilizer, landscaping and bedding.
Vermifiltration is one of many additional approaches and, Kirk noted, one used on MSU campus to reduce food waste and create a "really good, high value compost and fertilizer".
Red worms work through the manure, vastly reducing the ammonia content of the wastewater while making saleable worm casting by-products.
Evaporative systems go a step beyond simple drying, evaporating the liquid through a series of cascading evaporative reactors. The resulting solids can be used as bedding, compost and even manure composite boards.
Although manure application costs are still inexpensive, Kirk pointed out, social pressure is extremely high. "We know, socially and regulatory, there's a lot of eyes on how we manage manure. It is something that we, as an industry, need to be prepared to address."