Pigs on a factory farm.

Not surprisingly, the pig farm was an interesting contrast to my visit to a municipal wastewater treatment plant. This particular farm had 30 separate buildings that each housed about 1000 pigs. The pigs in each building were approximately the same age and raised together, growing from birth to a slaughter weight of 110 kg (220 lbs) in 160-165 days. Each sow has two litters per year. At any one time there were 30,000 pigs onsite; in one year, the facility rears 60,000 pigs.

The nitrogen and phosphorus in the urine and feces generated by 30,000 pigs are equivalent to about 75,000 people. Volume-wise, the pigs on this farm created 60,000 m3 of manure each year, enough to fill 24 Olympic-size swimming pools.

So why do I care about manure?

An algal bloom like this could ruin a day at the beach.

For two reasons. First, curiosity; as a consumer, I want to understand the food production system. Second, as a limnologist, I care because manure is one of those things that can "roll downhill" and can end up in a nearby lake or stream. Manure is rich in nitrogen and phosphorus, (as well as fecal bacteria and pharmaceutical residues). Nitrogen and phosphorus are important crop fertilizers and have a similar effect in lakes and streams, causing excessive algal blooms that, in turn, can reduce water clarity, reduce oxygen concentrations, and cause odors.

In many areas of the US and Europe, modern agriculture is highly specialized – focusing on specific crops (e.g. maize) or specific livestock products (e.g. eggs, meat). Because of this specialization, livestock producers may have little or no land to grow feed for their animals. Manure is heavy and not cost effective to transport it very far. But to “get rid" of manure (because it has to go somewhere), it can be applied in excess of what the crops need. In such cases, nutrients can accumulate in the soils and leak into surface- and ground-water.

Rules about manure handling and fertilization rates vary widely between countries and US states. Some areas limit the number of animals that a farm can have to prevent over-fertilization. Some areas provide financial subsidies to farmers to cover the cost of manure transportation. Some areas limit how much and when manure can be applied to cropland.

Farm tour

We didn’t see the stables because of strict hygiene and biosecurity standards. The pigs are kept in high densities and there is risk that visitors could introduce a virus or bacteria that could sicken the animals. Workers are required to go through an intense disinfecting process when they enter and leave the stables to minimize the risk of disease transmission.

Floors in the stables have slats, so that urine and feces fall though. This farm recently changed how it handles manure, so I will compare the old and the new process. For many years, gravity took the manure slurry (mixture of solids and liquids from urine and feces and water used for cleaning) through pipes to an open lagoon, basically a small lake. In flood-prone areas, storms can flush manure out of these lagoons into downstream waters. Indeed, this happened in the US after Hurricane Matthew.

Manure lagoon.

The centrifuge removes more suspended solids after the slurry passes through the screw press.

Once in the lagoon, manure slurry was pumped into large tanks that were pulled by tractors and sprayed onto fields. But solids would separate and settle to the bottom. The size of the lagoon made it difficult to mix the slurry in order to evenly distribute the nutrients (and liquid and solids). It was also time-consuming and expensive to use manure from the lagoon because the tractor would need to fill up a tank, spray the slurry on the fields, and then drive back to the lagoon to refill the tanks again and again As a result, the fields closest to the lagoon tended to receive more manure nutrients (especially phosphorus) than needed by crops.

One of the challenges of using manure as a fertilizer is that livestock produce it year round, but crops only need nutrients during the growing season. To better match the supply of nutrients from animals with the demand for nutrients by crops, farms need manure storage.

The solid portion of manure removed by the screw press drops through openings in the floor into an open truck bed.

After the screw press, the remaining liquid enters a centrifuge that further removes suspended solids. These solids also fall to the same open truck bed below.

Under the farm's new manure management approach, slurry from the pig barns is pumped into covered, temporary storage tanks. Before the manure is used as fertilizer, it is further processed. First, the slurry enters a machine (called a macerator) that removes "chunks" and large objects that could damage downstream equipment. Next, the manure enters a screw press, which separates the solid and liquid portions.

Acidificiation reduces odors and helps retain nitrogen.

Once per day, the truck takes the solid manure fraction removed by the screw press and centrifuge to a nearby farm, where it is used as fertilizer. This solid fraction contains about 85% of the phosphorus from the original manure slurry.

Sulfuric acid is then added to the liquid manure that leaves the centrifuge. Lowering the pH of slurry reduces odors and helps keep nitrogen from escaping later.

Lastly, the acidified, separated liquid fraction is pumped into long-term storage tanks. These tanks look like football field-sized, in-ground pools. The tanks are covered with thick, flexible rubber to prevent leakage and dilution by rain and snow.

The liquid manure fraction is applied to the farm’s fields using an extensive system of hoses connected to a tractor. Hoses transport the manure up to 6 km from the storage tanks to the fields. Spreading equipment consisted of dribble bars that apply liquid slurry to the soil surface. This method reduces the risk of leakage to the environment. Unfortunately, the weather was not cooperating, so we couldn’t see how this system works.

With this new method, the farm aims to use manure more efficiently so they can reduce phosphorus surpluses in their soils, reduce losses of nitrogen during storage, and reduce fertilizer purchases.

It is probably an occupational hazard, but now I think about manure management not only when I am at the office, but when I am at the grocery store as well.

Text: Michelle McCrackin