Technically speaking

Black and Veatch's Frank Rogalla looks at thermal drying of wastewater biosolids

Dryers at Ford STW use methane from the digesters

Dryers at Ford STW use methane from the digesters

As agricultural resistance to the use of treated sewage sludge grows and burial is prohibited by the landfill directive, heat drying, also known as thermal drying, is increasing in popularity as a way of managing wastewater biosolids.

"As regulation tightens in the US and European Union, more thorough treatment of biosolids for pathogen destruction prior to recycling to the land is required," according to Black & Veatch residuals management director Gary Shimp. "Heat drying essentially eliminates pathogens in biosolids, so some utilities are looking at drying as a means of upgrading the current level of treatment and transforming biosolids into a form that has more diverse uses," he continued.

The Escambia County Utilities Authority (ECUA) in Florida, USA, needed to modify the operation of its wastewater treatment plant due to complaints about an incinerator from the surrounding community. For the first time in a US municipal application, fluidised bed drying technology was selected for the Pensacola plant because of the technology's economy at the planned scale and pellet-sized final product. The pellets from the steam-powered heat dryers contain only 3 - 4% water. A distribution and marketing program targets local agricultural users, as the utility is selling the dried product to help offset some of the transportation costs.

When Kentucky's (USA) Louisville and Jefferson County Municipal Sewer District wanted to tackle odour and solids processing problems at its main treatment plant, a $6.4M design and build joint venture project was set up to design and construct a new process to treat 305t/d (dry) of raw primary solids and waste-activated sludge at the plant.

The 43-year-old, 400Ml/d plant is located less than 17km from Louisville's downtown business district in a largely industrial sector adjacent to residential areas. In addition to treating its own solids, the plant processes solids imported from other facilities, making the site a lightning rod for neighbourhood activists concerned about odours. An Alternative Solids Process replaced the low-pressure oxidation Zimpro system that had been in use, and a primary source of annoying odours, since the 1970s.

During project planning, existing structures were re-used to house the process, totally enclosing the dryers to minimise odour and reduce construction costs. Four existing anaerobic digesters were updated and restored instead of building new ones. In addition, the project team scaled-up heat drying technology to minimise the number of dryers required, contributing to procurement and construction savings.

To optimise the project's long-term sustainability, plans call for using methane gas produced in the anaerobic digestion process to fuel the dryers. The dried biosolids pellets that result from the process can be used as fertiliser or, when needed, be safely disposed at a commercial landfill, reducing landfill disposal volume by 50%.

To comply with EU regulations, Southern Water has built a new wastewater treatment plant at Ford, near Arundel in West Sussex (WWT, December 2001). The new plant, completed in July 2001, treats the wastewater from a combined population of approximately 140,000 from two towns. Treatment is to secondary level and enables Southern Water to meet the requirements of the EU Bathing Water Quality Directive.

Sludge from the treatment works is thickened and digested in three anaerobic digesters. Methane generated in the digestion process is stored and used to heat the incoming thickener sludge. Digested sludge is dewatered by centrifuges before transfer into the thermal dryer. The dryer uses methane gas from the digesters as its primary energy source with an alternative supply of natural gas from the town supply. With the addition of sludges imported from other works in the area, the total sludge treatment capacity at Ford will be 9,000t/pa of dry solids.

Because the Ford works is located in an area subject to strict planning review and regulations, the new facilities employ complete odour control and meet strict noise control requirements.

The new 55Ml/d plant at Kinneil Kerse provides East of Scotland Water (EoSW) with increased treatment capacity and ensures a final effluent that complies with both the Scottish Environmental Protection Agency's and the EU Urban Wastewater Treatment Directive's requirements for the secondary treatment of wastewater before it is discharged into rivers and coastal waters. The main plant was built on a site adjacent to the existing plant, which was later demolished. The work also included refurbishment of two existing pumping stations, a new pumping station and conversion of an existing treatment plant works to a pumping station.

The main wastewater treatment process consists of inlet screens, de-gritting, a storm tank and sequencing batch reactors. Sludge will be dewatered, then thermally dried. The new plant will also include facilities for processing imported cake and sludge. The thermal dryer converts dewatered sludge of 18 - 20% solids into 4 to 6mm-diameter granules comprised of 92% solids. The dryer has a water evaporation rate of 6t/h. "This project has improved the quality of effluent released to the environment," said EoSW investment planner Michael Breingan, "and the granules produced at the new sludge treatment centre can be reused in a number of beneficial ways, including for forestry, land reclamation and energy."


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