Tree shaded parks, roads and houses, flowing lawns, a profusion of flowering and flamboyantly foliaged shrubs; Walla Walla is green.

This abundant vegetation has many obvious benefits: tree-shaded roads reduce the urban heat island effect and the need for household cooling in summer, and make living, working and walking in the city far more pleasant. Since 2007, the Walla Walla area has had further benefits from all the greenery in and around the city — a profusion of locally generated and processed greenwaste compost at the city’s Sudbury Landfill.

The blizzard of leaves that descends annually on the city in fall, tree trimming, yard waste, lawn clippings, and grape pomace and stems from the winemaking process generated 5,362 tons of greenwaste in 2017, and in 2018, 6,538 tons (13 million pounds). Diverting this amount of compostable waste saves a huge amount of airspace in the landfill, greatly lengthening the amount of time it is usable, and additionally helps minimize the amount of methane generated when the landfill areas are sealed and entombed (capped). This locally generated and produced compost also has value to us individually, and to the city as a whole.

Most compost is sold locally, but some is sold for large highway projects or as far away as Seattle. Customers can buy a 5-gallon bucket full or large truck loads. Some custom screening is available for very large orders but must be planned for well in advance of need. A soil/compost blend is also available for raised boxes. If using the compost for a vegetable garden, supplemental nitrogen should probably be included as the compost is low in nitrogen.

Compost, when used as a top dressing or incorporated in the soil, increases the amount of organic matter in it and thus improves the structure, porosity and water infiltration, allowing better penetration and percolation of rain and irrigation water. It also increases the soil moisture-holding capacity, making plantings more resistant to drought, reducing water use.

Compost provides slow release of nutrients and minerals to the soil and plants and boosts and feeds beneficial soil organisms that naturally cycle nutrients, aiding to avoiding or limiting the use of fertilizers that may end up in waterways or groundwater. It can be used as a top-dressing or mulch under trees, shrubs or perennials. It is greatly beneficial as a top dressing on lawns.

We may think of compost-making as a basic process where our yard waste is mixed together, kept moist and breaks down over time, but making it commercially is both an art and a science. The process must adhere to the landfill’s set operational plan and Washingon State Department of Ecology’s compost regulations. The entire process is monitored by the state Department of Health and the state Deptartment of Ecology.

When greenwaste is brought up to the landfill, it is segregated into piles. Higher nitrogen and noncoarse greenwaste like leaves and grass clippings are separated from coarse, high carbon and lignin greenwaste like tree branches and trimmings. New and old piles are also kept separate. Twice a year the greenwaste is ground with a large grinder: in October before the leaves fall and in April. As the grinding machines cost upwards of $300,000 and require frequent and expensive repairs, the city of Walla Walla has elected to contract this part of the process out.

Greenwaste is fed into the grinder with a managed plan, mixing soft and coarse materials and old and new to generate a consistent end product with a ratio of three parts carbon to one part nitrogen. It is imperative that the material in the piles is coarse enough for air to easily percolate through as the composting process is based on aeration. The microbes that break the green material down need oxygen as much as we do, and the more available to them, the faster and more consistently the process works. The soft, higher nitrogen material provides immediately available “food” to the microbes that begin breaking the green material down, and this action causes the piles to heat up. Coarser material takes longer to decompose and provides long-lasting organic matter. Green waste is initially ground to about 3-4 inches in size, and after the composting process is complete, finished compost is screened to ½ to ¾ of an inch.

Trash is pulled out of the piles as it comes in, and screening reduces the presence of smaller pieces. It is extremely important for green waste to come to the landfill free of trash and even free of bits of small plastic like the green plastic pruning ties and plant tags so frequently used. Natural twine is a good alternative.

Temperature is extremely important in the composting process. The windrow-processed compost must get to at least 131 degrees 15 times to kill pathogens and weed seeds. If temperatures rise too high — above 170 degrees — the heat kills the organisms that cause it to decompose. Piles are carefully monitored with a 3-foot-long thermometer pushed into the piles and are mixed and turned to keep temperatures at desired levels and consistent throughout.

Constant temperature monitoring is also essential for two required state Department of Ecology regulations: the Vector Attraction Reduction test and the Process to Further Reduce Pathogens. Windrowed compost much reach a 114 degree threshold for 15 days, and the Aerated Static Pile system for three days.

The composting operation uses two methods of making compost. A more basic, time-consuming and space-intensive method is the windrow system. In this system, ground green waste is made into long windrows about 20 feet wide and 100 feet long. During 90 days of heating and turning, the piles must reach 131 degrees for 15 days and must be turned at least five times. After completing 90 days of heating and turning, the compost has an additional 30 days of sitting to cure. This final phase is when both actinomycetes (a fungi-like bacteria) further break down the coarse material and release plant nutrients. The white material often seen throughout the piles are actinomycete filaments.

The second and newer method, is called the Aerated Static Pile system. Four-inch HDPE pipes with holes drilled in one side, (like PVC pipe, but rated for high heat and also nonbrittle/nontoxic and more durable) are laid on pavement with the drilled holes on the bottom. Ground green waste is placed on top of the pipes in windrows about 80 feet long, 40 wide about 10 feet tall. Piles are capped with finished compost to hold in heat and inoculate piles with beneficial organisms. A 1.5 horsepower Jumpy house fan forces air through the pile at timed intervals of about 1 minute every 20 minutes to keep everything oxygenated. At the end, aeration pipes are pulled out with a loader.

The benefits of this new system are that the method only takes 30 days of air being forced through and 30 days to sit and cure — a total of 60 days from start to finish compared to the windrow system that requires 120 days for processing and curing. The ASP system saves time and space, fuel, and labor costs. There are eight of these ASP systems active now, each costing about $1,500. The city plans to convert more of the operation to this system.

When finished, each batch of compost undergoes extensive testing. Nutrient levels as well as testing for maturity, herbicides, weed seeds and pathogens is standard. A typical bioassay test grows cucumber seeds in the compost to test for the presence of herbicides, compost maturity and appropriate carbon and nitrogen content. Tests are available on request.