Local conditions save on primary energy
Regulations in most European countries stipulate a specific ranking for methods of organic waste treatment.In terms of municipal sewage sludge, priority needs to be given be given to utilising the substances it contains. Sewage sludge can, to a limited extent, be used in agriculture - as organic fertilizer. If this is not possible it is recommended to make thermal use of the waste as a fuel after it has been suitably treated, i.e. thermally dried. As a last step, residual waste can go to landfills after its organic content has been reduced to below 5%, or for some landfills below 1% - ash from thermal use of sewage sludge generally meets this.
Long term disposal
Reliable disposal in the long run can only be achieved by a combination of agricultural and thermal use. Thermal use, as stipulated by the law, does not necessarily mean that organic waste should be destroyed by heat, but that it should be processed to solid fuel and then used instead of primary energy. The idea is also to reduce CO2 emissions. Relatively safe use in agriculture and proper thermal use is only possible if the sludge has been dried, normally to a dry solids content of 90- 95%, by heating to more than 80°C for a minimum of 20 minutes in order to pasteurise the granulate.
This is why there has been a veritable boom in sewage sludge drying plants over the past few years. In larger drying plants, drum drying equipment now leads the field. Here, dewatered sludge is first fed to a mixer where it is mixed with dried material to generate a moist granulate mixture which is no longer sticky. This mixture is dried to 95%DS with hot air in the triple-pass rotating drum. Since the material remains in the drum for 20 minutes and reaches a temperature of 80-85°C, the granulate produced meets all the hygienic requirements.
The fine product (<0.8 mm) from the drum is returned to the mixer. The final product is suitable for long-term storage without the risk of unpleasant odours developing and can be put to a multitude of uses This most common method of drying requires a high thermal energy input. Also, the process releases additional CO2 as primary energy is used.
Over the past 5 years, Andritz has built a number of drying plants for municipal sewage sludge which tread new ground in terms of thermal energy. Two projects demonstrate how local conditions can be fully utilized to save primary energy.
Combustion gas use
The sludge drying plant at Bran Sands in the UK uses combustion gases from gas turbines for heating instead of primary energy.
The Bran Sands plant is a treatment facility for municipal sewage sludge located on Teesside in northeast England, built on land previously used by heavy industry. The plant handles an annual volume of 1M metres3 of wet sludge, approximately 50,000 tonnes DS. The sludge is not digested and currently is mainly primary. By the time the local sewage treatment plants will have been extended - i.e. over the next 5 years - the operator expects to receive more secondary sludge. Storage is followed by:
- a cleaning stage using strain presses;
- a dewatering stage with a total of 14 belt presses, each with a belt width of 3 metres and dewatering to 25-28% DS;
- a drying plant with a total of 7 dryer lines, each evaporating 5 tonnes of liquid per hour;
- a pelleting plant for pellets which are 4-12 mm in diameter;
- a silo plant (20 silos each holding 100m3 granulate and/or pellets;
- a loading plant (2 lines operated independently in parallel).
- 2 parallel gas turbines (fired with natural gas, 5MW each), supplying hot exhaust air (around 430°C) as heating energy to the dryers and, more importantly, producing electric current supplied to the national grid;
- a complete sewage treatment plant for the filtrates produced in the sludge dewatering stage.
Four of the seven dryer lines at Bran Sands are directly heated, exclusively with hot gas from the turbines. Small quantities of primary fuel are required for inerting. The initial phase handling some 50% of the sludge has been in operation since mid-1998. The second phase went online in 2001. In the next stage, it is planned to convert the dried sludge in gasification plants and to use the biogas produced there to replace the natural gas for the turbines.
At Obrigheim in Germany, raw sludge is dried in a fluidized bed system to 90% dry granulate which serves as fuel in the attached cyclone burner. The system processes the sludge from a population equivalent of 100,000, evaporation rate is 565 litres of water per hour. The most economical sludge treatment method was determined by a local engineering consultant and was found to be a combined drying and incineration system.
Principles of EcoDry After the sludge has been dried in a fluidized bed or drum dryer, it is burnt in the primary chamber of the cyclone furnace. The hot combustion gases are routed into the secondary chamber of the furnace for afterburning. The ash goes to the ash chamber. The combustion temperature is controlled by adding cooling air, which prevents the major part of heavy metals in the sludge from entering the gaseous phase. They are discharged together with the ash.
The emission parameters achieved at Obrigheim are considerably below the limit as per 17th BimschV (German Regulation on exhaust from hazardous waste incineration plants). The lowest annual total cost is achieved with a solid fuel heated dryer such as the EcoDry.