Mobile decentralizes to accommodate growth
Earlier this year, the Mobile (Ala.) Area Water and Sewer System (MAWSS) completed construction of the Hutchens wastewater treatment and disposal facility. Operating independently of the MAWSS central treatment plant, Hutchens incorporates sand filters and soil-based treatment systems to manage wastewater for a newly constructed neighborhood. It is one of several cluster systems installed by MAWSS to accommodate Mobile’s growing development and treatment demand.
The decision to build a decentralized treatment plant was prompted by two factors: The MAWSS’s need to expand treatment capacity to accommodate a new school and 80 homes on the city’s perimeter; and its need to expand quickly rather than lose the new business to a nearby rural competitor. Working with locally based Volkert & Associates, the utility designed and constructed four neighborhood-based systems that could be installed in three to six months. Furthermore, the systems could be installed for substantially less money than it would cost the utility to develop a new centralized treatment plant.
Hutchens, which treats up to 240,000 gallons of wastewater per day, was the first such cluster system to be completed under the plan. Built on land adjacent to the school, the two-acre plant includes a recirculating sand filter for pretreatment, as well as two soil-based treatment systems. It connects to 80 homes and can accommodate 1,072 more.
Wastewater is collected in 1,000-gallon interceptor tanks connected to each house, and solids are removed via anaerobic digestion. Effluent is pumped to a soil-absorption system supplied by Old Saybrook, Conn.-based Infiltrator Systems. The polished effluent is stored onsite (in a 1.5 million gallon pond) and used to irrigate a 30-acre sod farm next to the school or to recharge the groundwater.
The Hutchens system took six months to design and build, costing the MAWSS $710,000 for construction and related infrastructure improvements. Developers pay the household connection fees ($1,762 per home), allowing the utility to recoup its capital outlay for the project.
As manager of the new facility, the MAWSS has executed a homeowner maintenance agreement that limits household discharge of grease and oil and restricts use of garbage disposals. The utility installs a monitor on each interceptor tank, and an alarm sounds if grease content exceeds the acceptable level. Homeowners pay for the utility to pump the tank and remove grease.
The Hutchens facility is removing more than 90 percent of carbonaceous biochemical oxygen demanding substances, which affects the system’s ability to remove nitrogen. Therefore, the design consultant is reviewing operation data and adjusting operation of the recirculating sand filter to improve nitrogen levels.
The Hutchens facility has been recognized locally by the Mobile Area Council of Engineer’s and the Alabama Society of Professional Engineer’s as both the Mobile Area and the State of Alabama 2001 Engineering Project of the Year. Based on the success of the cluster systems the MAWSS plans to divert wastewater from existing sewer interceptor lines and to add treatment facilities for soil-based reuse and groundwater discharge. Ultimately, the utility expects the decentralized cluster systems to meet all the area’s residential and commercial wastewater treatment needs.
