The moisture content of biosolids produced by mechanical dewatering equipment often restricts the feasibility of pursuing alternate biosolids utilization options or makes existing utilization plans inefficient. Increasing the dry solids content of dewatered biosolids usually decreases the volume and always decreases the mass of material to be handled in the next step of the biosolids management program. As the treatment and elimination of biosolids represent over half of the capital and operating costs of most municipal wastewater treatment plants (WWTP), the savings can be substantial.

When designing a biosolids management program, it is customary to evaluate and select the preferred ultimate biosolids utilization method. The ultimate use will dictate the dry solids requirement of the biosolids and this will in turn dictate the types of sludge dewatering equipment, and biosolids stabilization processes, which can ultimately be selected. For example, composting is most efficient in terms of capital and operating costs when the biosolids have a dry solids content of 30 to 35%.

When the biosolids are wetter than this, the moisture and energy balance must be supplemented through the use of increased woodchip amendment and compost recycle. Likewise, incineration is most efficient when the sludge burns autogenously, without the need for supplemental fossil fuel for combustion. This usually occurs when the dry solids are greater than 35%. While current biosolids dewatering technologies, such as high solids centrifuges and diaphragm recessed plate filter presses can achieve 35% solids, the majority of biosolids dewatering facilities currently in operation utilize belt filter press technology which typically achieves 16 to 22% dry solids in municipal WWTPs.

In a new application of an existing technology, the biosolids can be further dewatered in a thermal dryer to augment existing or proposed dewatering facilities. Thermal dewatering can be used to produce a product at the optimum dry solids to efficiently satisfy the biosolids utilization plan.

Historically, biosolids dryers have been used to produce a pelletized product at 90 to 95% dry solids. The pellets have been used as fuel or low grade fertilizer. The majority of dryers use convective, or direct, heat passing hot dry air directly over the dewatered biosolids as they tumble through a rotating drum.

Recently, conductive, or indirect, dryers have been gaining popularity. The indirect dryers differ from direct dryers in that heat is transferred to the biosolids by conduction through a metallic surface. The surface may be the wall of the dryer, hollow screws, paddles or disks. The interior of the heat transfer surface is usually heated with hot oil or steam. The advantage of indirect dryers is that, as the heating medium is not in direct contact with the biosolids, there are minimal off-gases to treat when compared with direct drying.

Some types of indirect dryers can receive biosolids directly from the dewatering equipment, at as little as 10% dry solids, and produce a thermally dewatered sludge cake at any dry solids content up to 95%.

At the Buffalo Sewer Authority's Bird Island Sewage Treatment Plant, CH2M Gore & Storrie, in conjunction with Nussbaumer & Clarke, designed and commissioned North America's first full-scale installation of thermal dewatering technology on municipal biosolids.

Commissioned in 1992, the installation uses an indirect dryer consisting of a series of hollow disks heated internally with steam and mounted on a single rotor. The unit, manufactured by Stord Inc., dewaters 100 wet tonnes per day of biosolids produced from the belt presses, increasing the dry solids content of the biosolids from 18% to 35%.

As dewatering occurs in the unit by boiling off the water in the biosolids at 100 degrees Celsius, some 50 tonnes of water per day are boiled off in the dryer. The small non-condensible gas stream remaining, after the resulting water vapour is directed to a venturi type condensing unit, is oxidized in the incinerator. By way of comparison, the 50 tonnes of water driven off in the dryer would otherwise have been vaporized and heated in the incinerator, through energy supplied by natural gas, to 600 degrees Celsius.

The cost of the indirect thermal drying system, including construction and equipment was $995,000 (U.S.). With a payback period of less than six years, this innovative project has resulted in similar installations in three other American cities. With over two hundred biosolids incinerators operating in North America, there is great opportunity to produce energy from autogenous combustion of biosolids from a process which is generally a consumer of valuable energy.