Biosolids as fertilizer: two birds, one stone

Ron Muzzall grows alfalfa and grass as part of a dairy and multiple crop farming operation. His farm is similar in many ways to others, except that certain fields are fertilized with biosolids from a septage treatment facility.

Island County, Wash., built the facility to treat septage collected on Whidbey Island, a large island in Puget Sound, north of Seattle. The island is largely rural, and many residents use on-site sewage disposal. Because most of the people get their water from a sole-source aquifer, Island County is concerned about water quality and has taken a proactive approach to managing on-site sewage disposal systems.

Neither the small publicly owned treatment works on the island nor the larger regional plants were interested in accepting septage, and the county embarked upon a facilities-planning process to determine how to treat and dispose of septage. Because of state disposal regulations covering septage, the only economic means of managing septage was by treatment and utilization.

The facilities plan considered a number of different treatment processes, and identified aerobic digestion and lime stabilization as the two most desirable.

The county built the facility, one of very few plants nationwide that treat only septage, in 1994. In fact, one of the challenges the Seattle-based R.W. Beck design team faced was the lack of similar ideas to use in its design. Team members did not want to. be pioneers, but instead were looking to use proven and practical technology. Although unproven for treating septage, the team chose to break new ground and use the aerobic digestion process for the Island County facility, using data developed by Island County in a winter/summer pilot project carried out in 1992. The process was selected because it. produces an inoffensive, biologically stable end-product low in volatile solids, has a lower potential for creating odor problems during storage and use and produces a liquid product suitable for irrigation. Despite its higher cost, the process results in an end product better suited for reuse on Whidbey Island.

The plant is designed to treat an average of 5,100 gallons of septage daily. Major treatment units include a mechanically cleaned bar screen and screenings press, an aerated grit chamber and two aerobic digesters. Storage lagoons with geomembrane liners store treated biosolids and digester supernatant.

The county applies the supernatant to forestland adjacent to the facility. To control odors, the screen channels and the aerated grit chamber are covered to collect odorous air for treatment in a biofilter.

The biological process involves diffusing air into a mixture of septage and bacteria to provide oxygen to support the microorganisms and provide energy for mixing. There are two digester tanks, each with a capacity of about 167,000 gallons.

The facility produces Class B biosolids that must be stabilized to meet vector attraction reduction requirements and must have a pathogen concentration of less than 2 million fecal coliform organisms per gram of dry solids to comply with federal regulations. The biosolids produced in Island County’s aerobic digesters have an average fecal coliform content of 11,000 organisms per gram and have easily met the stabilization requirements.

To combat odors, the project team designed a biofiltet that works through an inexpensive, natural process. The plant’s odorous air is pumped into a mixture of half high-grade yard-waste compost and half wood chips. The biofilter has proven very effective for removing the odors associated with raw septage, and the entire plant has been virtually odor free.

The treatment facility is just the first step in Island County’s plan. After the septage is processed, the resulting biosolids are used as fertilizer/soil amendment for crop lands and forestry areas. County personnel worked closely with Ron Muzzall, a local farmer, and the Washington State University Cooperative Extension Service to establish application rates, methods and schedules for applying the biosolids.

Last July, Muzzall applied about 350,000 gallons of treated biosolids to his land. In two months, the field produced two cuttings of alfalfa — more than Muzzall might otherwise have harvested that summer.

Implementing the reuse program has gone smoothly, says Dave Bouvouloir, Island County’s solid waste manager. “We were ready for opposition, but it hasn’t materialized,” he says. “People on Whidbey Island are more attuned to farming and its methods than folks in more urban settings.” Bouvouloir attributes the lack of opposition to the fact that the public was involved in the process from the beginning.