Sludge disintegration is rapidly becoming a valuable area of wastewater treatment and it is a significant development in environmental terms. The UK is second only to Finland in terms of waste disposal to landfill but supply of holes in the ground is finite.
Costs of disposal to landfill are increasing and farmers can only take so much sludge for agricultural purposes. Most products in the wastewater industry are purchased to
comply with legislative requirements or to increase operational efficiency.
The Biogest Crown Sludge Disintegration System can do both, it can reduce overall final sludge volumes for disposal and improve dewatering. In addition, it can provide an improved income stream by increasing the biogas yield from digested sludge.
One of the principle benefits of the Crown Sludge Disintegration System is the 20% reduction in waste biosolids volumes that is a significant step forward in reducing waste to landfill. The other main attraction of sludge disintegration is that it will increase biogas production at the digester stage, typically by 30%, generating energy from a waste source and a revenue stream from the sale of electricity.
Anticipated paybacks on this basis are expected to be around 12-24 months. While sludge volume reductions and increased biogas yields alone make sludge disintegration a good investment, there are numerous additional operational benefits. Improved dewaterability is one specific area, along with a saving in the use of flocculants.
The destruction of fillamentatious bacteria reduces or eliminates foaming in the digester and provides a generally more stable process.
Sludge disintegration systems can be used in a variety of applications.
- Treatment of secondary sludges prior to digestion, giving rise to the following benefits:
- reduction in sludge volume (typically 20%+),
- increase in biogas yield (typically 30%+),
- improved dewaterability (typically 5%DS),
- improved stability of digester,
- reduced foaming,
- pathogen reduction.
2. Treatment of return sludges, giving rise to the following benefits:
- destruction of fillamentatious bacteria, reduction in bulking sludges. There are a number of systems for disintegrating sludge, those based on ultrasound perhaps the most well-known. There are various manufacturers of ultrasound systems but the basic principle uses ultrasonic transducing heads to create sound waves above 20kHz. When sludge comes into contact with the ultrasonic head, the sound wave causes
bacterial and organic cell cavitation in the sludge. This process creates
huge pressures and temperatures up to 5,000K.
Clearly, this is a very aggressive environment and creates problems with material selection. To withstand the harsh environment, the ultrasonic heads are now being manufactured in titanium. However, even titanium heads have a limited lifespan in these conditions.
At Wiesbaden in Germany an ultrasonic disintegration system was installed to handle a sludge throughput of 4m3/hr but after just six months in operation, the sonic heads
had to be replaced at a very significant cost.
Clearly, this sort of operational expenditure will make a dent to the income from the additional energy sales.
Biogest of Germany, through its long-term distributors CSO Technik, are announcing the UK launch of an alternative sludge disintegration system that eschews ultrasonic technology for a simpler and cheaper mechanical process that delivers similar performance to ultrasonic-based systems. Again, the system causes cell cavitation within the sludge, but as the reaction takes place within the flow rather than on the surface of any material, the whole system can be manufactured in
stainless steel without any detriment to performance.
Although the Crown system is mechanical rather than ultrasound-based, the principle of cavitation remains the same albeit by a different methodology. The mechanical cavitation is both stable and non-sensitive to variable sludge loadings. Cavitation releases endo and exoenzymes and destroys fillamentatious bacteria. The increased biological activity is directly responsible for the reduction in solids, after digestion, and the increase in biogas yields. The destruction of fillamentatious bacteria leads to a reduction or elimination of foaming in the digester and consequently reduces the demand for flocculants.
The system can treat all or just part of the sludge going to digestion, depending upon the client’s requirements. These requirements may be dictated by operational or financial considerations, or a combination of the two. Monitoring of the initial trial plant and operational systems would seem to indicate the best financial return can be achieved by treating just 30% of the sludge volume for digestion.
The Crown system is a multi-stage system. The two principle components are the homogeniser and the disintegrator. The Crown system consists of a homogeniser, two pumps, recirculating tank, Crown disintegrating nozzle assembly, connecting pipework and a control panel. This is normally supplied as one assembly either skid-mounted or in a container.
The footprint of the Crown system is considerably smaller than other disintegration solutions – a recently installed plant handling 60m3/hr at Wiesbaden has a footprint of a similar size to an adjacent ultrasound plant handling just 4m3/hr. The Crown system is prefabricated on a skid or into an ISO container.
Once delivered to site, the unit simply requires to be connected to a power supply and the sludge inlet and outlet pipework. Sludge is initially fed into the homogeniser that renders the sludge to a uniform consistency. From there the sludge is pressurised using an eccentric worm drive pump up to 12Bar. The sludge is then forced under pressure through a nozzle and this causes the cavitation. Depending on the type of application, this action may be repeated to achieve the best results by recirculating the sludge via a holding tank.
The treated sludge is then fed to the digester using a second eccentric worm drive pump operating at 4Bar. The eccentric worm drive pumps are fitted with pressure regulators to accommodate variable sludge viscosity and wear in the pump. This ensures the cavitation process is stable and non-sensitive to variable sludge loadings. The control panel incorporates a human machine interface (HMI) in colour that makes operator use simple and user-friendly. The programmable logic controller (PLC) is a Siemens SPS (S7) that links to the main plant motor control centre (MCC). One of the principle benefits of the Crown system is its simple robust engineering. All of the components are tried and tested and no specialist maintenance is required. Operating costs are low, both in terms of electrical requirements and the annual spares costs. There are seven operational plants installed, treating from 5m3/hr up to 60m3/hr with a further large system under production for a scheme in Kappala Sweden (PE 750,00). Data from the installed plants shows average solids reductions of 20% and increased biogas yields of 30%.
All plants are reporting improved dewaterability of sludge. CSO Technik’s UK clients are being offered a unique opportunity to try before buying. Biogest will build a plant specifically tailored to the client’s sludge treatment requirements.
The plant will be hired to the client for a nominal rent for a period of six months. If at the end of the rental period the client does not want the equipment, CSO will remove it at no additional charge.
However, should the client want to keep the plant, the rental payments will be deducted from the final invoice.
A manufacturer’s standard control panel will be supplied initially, which can be upgraded to a full WIMES specification panel at the end of the rental period. CSO Technik and Biogest have been working together since 1995. Initially, the relationship started as a simple distributor/manufacturer arrangement in the UK.
Today, CSO also has distribution rights for additional countries. CSO and Biogest have also carried out collaborative research on a new storm screen product conceived by CSO. The successful conclusion of this project has already led to several orders.