Lime Stabilization Helps Expand Biosolids Program

The City of Gastonia, NC, recently upgraded its biosolids treatment program as part of an expansion of its Long Creek Wastewater Treatment Plant to 16 mgd.

At the time of expansion, the city was faced with exorbitant costs to refurbish its anaerobic digesters. The city approached Synagro, a Houston-based Residuals Management Company, about ways to avoid the high capital costs while maintaining compliance with regulations.

Synagro proposed converting one 435,000-gallon concrete digester tank into a lime stabilization tank and using another digester as a holding tank for the stabilized biosolids. This eliminated the need to refurbish the digesters and still met regulatory requirements.

The Long Creek plant has a fairly typical activated sludge treatment process. Wasted sludge is stored in the former digester tanks until going through the stabilization process.

The process at Gastonia is unique, but simple. The stabilizing tank is equipped with a pump, which powers the 4 percent solids sludge through a "Y" shaped pipe at the top of the tank. Approximately 24 tons of powdered lime is pneumatically injected into the short end of the "Y", mixed with the sludge being pumped through the pipe to produce a "slaking" process (exothermic reaction), and sent back into the tank. Three mixers, each mounted to hoists, move up and down to thoroughly mix the contents.

At the end of the approximately two-hour period it takes to unload the lime, the required 12 pH has been attained and is maintained for an additional two hours. Twenty-two hours later the pH is checked for an 11.5 or greater. Once stabilized, the Class B biosolids are pumped into the holding tank, ready for land application.

There are two major benefits to this particular system: 1) it requires no additional water for the process, so there is no additional volume; and 2) the entire process takes little more than 24 hours. The city typically processes a batch twice a week.

Another partnered accomplishment by Synagro and the City of Gastonia is the creation of the Resource Recovery Farm. The city approached Synagro about using an idle 600+ acre tract of land, owned by the city, to meet some of its residuals management needs. The goal was to use the land for storage for increased volumes of biosolids and expansion of the land application program.

Synagro worked with the city to develop the farm which now consists of 200 working acres which produce corn, small grain, fescue, and orchard grass. The site includes two four-million-gallon lined storage lagoons; and a decant basin used for spray irrigation. The RRF has not only benefited the city, but also contributed to local, regional, and state institutions through research conducted on the farm.

The North Carolina Division of Water Quality, the Quality of Natural Resources Commission, Habitat Assessment and Restoration Program, Schiele Museum of Natural History, North Carolina State University, Gaston County Cooperative Extension, North Carolina Forestry Service, and the Natural Resources Conservation Service Division of Soil and Water Conversation have done research at the RRF.