BioCycle Magazine

Composting and local food merge st urban garden

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Growing Power (GP), a nonprofit urban garden and training center in Milwaukee, Wisconsin, provides affordable produce to neighborhoods without access to fresh food, and processes a variety of organic wastes through composting and anaerobic digestion. Located on a two-acre lot on Milwaukee's north side, the six greenhouses and several hoop houses include raised beds for herbs and greens, aquaculture, hydroponics, vermicomposting bins, an anaerobic digester, classrooms, kitchens and a retail store that sells goods from a network of 300 farmers. Outside are compost windrows and bins, goats, turkeys, ducks and a productive apiary of bees. About 100,000 pounds of organic produce are grown annually, and sold at the on-site retail store, through its market basket program, as well as to restaurants and food co-ops. GP has expanded its programs to Chicago, Illinois (see sidebar).

However, GP does more than just provide affordable fresh produce. All of the operations are set up to not only be productive, but as demonstrations - systems that can be easily replicated in other communities. “As more and more agricultural land is lost, intensive growing will be necessary,” says Will Allen, founder and CEO of Growing Power. “We produce about $5/square foot of produce annually in our beds, which translates to more than $200,000/acre. Now that is Growing Power.”

He purchased the urban lot for GP in 1993, returning to his farmer roots after careers of playing professional basketball, and later working for Proctor & Gamble. Being able to grow $200,000/acre is a powerful tool, one that is being taught in workshops at GP, as well as through collaboration with aid organizations such as Heifer International.

Among the dozens of workshops held at GP every year are lessons on how to build inexpensive aquaponic systems, the smaller of which only take a day. Aquaponics integrate elements of hydroponics and aquaculture, creating a symbiotic system for growing fish and plants where the fish waste becomes nutrients for the plants, which in turn purify the water for the fish. There are currently three tilapia systems and six yellow perch systems at GP, ranging in size from 250 gallons to 10,000 gallons (averaging about one fish per gallon). The yellow perch are a cold water system, and only require a small pump to move water up through tiers of vertical growing beds. The tilapia are a warm water fish, and therefore require a heater in Wisconsin. “Heating the water for our tilapia systems is more efficient than heating air in the greenhouses, so we don't have additional heaters for the greenhouses,” says Allen. “Some tanks are in trenches in the ground, utilizing geothermal heat.”

Water from the fish tanks is circulated through tiers of plants, going into gravel beds where bacteria break down the ammonia in the fish waste into nitrate and nitrogen. Watercress is planted on the gravel of one tier, which further filters the water. Another tier has tomatoes, which thrive on the nitrogen. Tomatoes, watercress and fish are all high-value cash crops sold at the retail store and to restaurants. “The system essentially mimics the biodiversity of a river,” notes Allen. It demonstrates high-efficiency production, as well as water conservation (water is only added when evaporated) and a nutrient management system.

Fish in the aquaculture systems are given conventional fish food, but Allen is experimenting with Black Soldier Fly larvae, which are 40 percent protein. “I discovered that the larvae are attracted to wet cottonseed, and that the fish love them,” he says. “The wet brewers grain that we compost also attracts the larvae, so we're trying to figure out how to breed them as a replacement for the fish food.”

Approximately 500 tons/year of organics are recycled at GP's urban center, and another 3,000 tons/year are processed at a 40-acre rural farm it operates. These materials consist of brewers grains, wood waste, preconsumer food scraps and coffee grounds, most of which are collected by GP (coffee grounds are delivered). At the farm, composting primarily takes place in windrows, with worms added toward the end of the process. At the urban garden, feedstocks are processed in vermicomposting bins, an outdoor windrow and in an anaerobic digester.

Vermicomposting is the primary method utilized for composting organics at the urban center, due to the nutrient value of the finished product and speed of the process. There are two stages: First, brewers grains and food waste are put into an outdoor static windrow, which is about 4-feet high and leans against the length of a greenhouse, providing heat for the building. Second, the material (once partially decomposed) is moved to a system of 50 vermicomposting bins inside the greenhouses, where it remains for 12 weeks. Tanks under the bins collect leachate, funneled to an aerated storage drum and used as compost tea. After 12 weeks, a household screen is placed on top of the worm bins with fresh compost on top, and the surface dwelling red worms crawl up and out of the finished compost.

The vermicompost is then shoveled into an old household clothes dryer modified to screen out rocks and other contaminants. “The heating element is often the first thing to go on dryers, with the motor for rotating still functioning, making them a common and useful tool for screening smaller amounts of compost,” says Allen. The compost is sprinkled on top of plants as a fertilizer, and blended for a potting mix with coir, a coconut by-product that is an environmentally friendly substitute for peat, in a ratio of 1:1. Some wood chips are left in the potting mix to allow for aeration and fungi development, which aid in nutrient delivery.

Two hoop houses at GP are heated entirely by the composting process, with active piles of brewers waste and wood chips in each of the four corners. Fresh material is added to the piles to keep them actively composting and generating heat. They provide enough heat throughout the Milwaukee winter to keep the hoop houses above freezing - warm enough to grow hardy greens like spinach and kale. Wood chips on top of the piles successfully filter ammonia. Run as a pilot project, an anaerobic digester in one of the greenhouses can process 5 tons/day of food waste. Eventually it will be used to power and heat the greenhouses. Material is recovered after the digestion process and used as a soil amendment.

Compost bins outside contain food scraps from various Milwaukee schools, mixed with yard trimmings. Schools bring classes out to the urban garden for lessons on vegetable growing, composting and other activities GP has to offer. Overall, Allen estimates that over 3,000 people tour GP every year, including attendees at over a dozen workshops on aquaculture and other operations. On our tour, Allen asks what we think the most difficult aspect of Growing Power is to manage. Stumping the group, he answers, “Managing and collecting waste. We strive to be zero waste, and we collect waste for composting, which means engaging with the community and establishing a reliable system for collection and composting.”
Collecting and managing organic waste streams may be the most difficult aspect of GP, but it is also one of the most important. Allen stresses that to maintain an intensive growing operation, with yields of $5/square foot, a nutrient replacement system is a critical component. The compost windrows, vermicompost bins and anaerobic digester are all part of the nutrient cycle.

As such, Allen has plans to expand organics collection with a 200 family residential pilot program, collecting food waste biweekly. The pilot, he hopes, will demonstrate to the city that residential organics collection can and should be instituted. There are also plans to turn part of GP into a five-story glass structure, to capitalize on vertical growing space, an increasingly popular concept for urban gardens and farms.

Two weeks before I toured GP, Allen won a $500,000 fellowship from the John D. and Catherine T. MacArthur Foundation, commonly referred to as the MacArthur “Genius Grant.” Visiting Growing Power explained why, with its maze of intertwined projects... the apiary of bees, the ducks and chickens, the vermicomposting, aquaponics systems and anaerobic digestion... everything is connected, making it difficult to differentiate where one project stops and another starts. Growing Power is a prime example of an urban garden using composting and other activities to cultivate more than just local food.

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