Digestion Plants

Anaerobic sludge digestion in wastewater treatment plants is widely used to stabilize concentrated organic solids removed from settling tanks, biological filters and activated sludge plants. Digestion renders the sludge suitable for reuse purposes such as land fill and garden mulch (local authority permitting). EPCO Australia designs systems and equipment for anaerobic processes such as:

Digester Roofs

• Buoyant gasholder
• Fixed steel
• Floating gasholder

Sludge Mixing Systems

• Lance mixing systems
• Shearfuser mixing systems
• Mechanical mixers

Sludge Heating

• Sludge recerculation type
• Hot water circulation type

Waste Gas Burning Systems

• Manual ignition, semi automatic ignition or fully automatic ignition.

Digester Roofs

In wastewater treatment plants, the digestion tanks are normally covered with concrete or steel roof structures of different shape and structural design. Dome shaped covers are often used both in steel or concrete construction.

Steel covers can be fixed or floating. Floating covers in primary digestion tanks have a number of advantages. The more uniform gas pressure produced by the floating roof provides a higher degree of operational control, resulting in more efficient operation of mixing, burning and sludge heating systems all of which use the pressurised gas.

Dry seal type gasholder covers are generally used in large treatment plants where gas storage is separate from the digesters. The piston is provided with a balancing system. The main seal is made from a special synthetic rubber. Concrete ballast is sized for the desired operating pressure. The gasholder is available as a single-lift or two-lift design. Gasholders may also be of the wet seal single lift type.

Sludge Mixing

Sludge mixing in the primary digestion tank is carried out to mix the incoming sewage with the digesting sludge and to maintain a uniform liquor temperature in order to optimise bacterial activity. Mixing also disperses any scum that may form on the surface of the digester liquor.

Mixing in the digester tank can be carried out by mechanical mixing or gas diffusion type mixing systems. Selection of the mixing system is dependent on the size and type of treatment system and the means of subsequent sludge disposal.

Sludge Heating

Heating of primary digesters maintains sludge temperature in a controlled range dependant on the sludge regime chosen (mesophyllic or thermophyllic). The heat exchanger can be located inside or outside the digestion tank. Internal heating systems are normally combined with the sludge mixing function of the tank as in internal or external draft tubes. Modern practice uses an external heat exchanger. Design parameters to be followed in sizing heat exchangers are of paramount importance, as overheating beyond narrow limits can cause perpetual sludge caking and associated maintenance and will inhibit the operation of the digestion process, whereas inadequate heating will not achieve the level of volatile solids reduction required.

Waste Gas Burning

Surplus gas produced in the sludge digester is burned to reduce plant green house gas emissions. The unique burning system of our Groth range of gas flares contain flame retention vortex vanes and when coupled with the downdraft preventer in the shield, results in an updraft and an air mixing action that provides a highly efficient, smokeless and odourless burn and virtually eliminates the possibility of the flame being blown out. The wind shield is designed to control outside winds up to 320km/h, and to operate efficiently in heavy rains.

Further info.