Sludge is produced during the sewage treatment process. It is then separated using either a decanter centrifuge or filter press, supported with an entire range of auxiliary equipment. A complete set of equipment for this operation is known as a sludge dewatering station. As a rule this means a separate building or enclosed space within the premises of the sewage treatment plant, where the following equipment is housed:
- mechanical equipment for dewatering the sludge (centrifuges, presses);
- system for making and dosing the polymer solution;
- sludge feed system;
- dewatered sludge collection system;
- extensive control and measurement system.
Because of the amount of equipment and its degree of complication breakdowns do occur in sludge dewatering stations, and it becomes necessary to plan equipment overhauls or modernisation. In times now thankfully behind us, when sludge dewatering stations were inoperable the sewage was discharged directly into rivers without the proper treatment. Such practice is of course unacceptable today.
It is at times of outage, breakdown or modernisation that Mobile Sludge Dewatering Stations are used. A Mobile Sludge Dewatering Station comprises a similar set of equipment as the stationary version, but is constructed in as compact a form as possible, and comes not in a free-standing building but in a truck trailer or articulated trailer. In addition it has to be designed to work not with just one but with many types of sludge. Including all the essential equipment comprising a Mobile Sludge Dewatering Station in a small trailer is a true challenge for engineer and designer alike, and demands substantial knowledge of the equipment itself as well as the dewatering process.
The following coefficients are used in the grading of dewatering equipment:
- the degree of dewatering – indicating the content of dry solids (dry residue held at a temperature of 105 degrees C) in the dewatered sludge, this value expressed as a percentage, e.g. a value of 25% DS means that after roasting at the given temperature, 75g of water evaporated from a 100g sample. The higher this value, the more efficient the equipment;
- polymer consumption – a flocculating agent, comprising long polymer chains, is used to prepare the sludge for mechanical processing; its consumption is expressed in kg of the agent used per ton of dewatered dry solids (kg/tDS). The more modern this equipment, the lower the polymer consumption;
- partition coefficient – a value defining what portion of the suspension contained in the sludge was extracted in the dewatering equipment and what portion was returned together with the filtrate to the treatment plant; this value is expressed in percentage, and for modern machinery amounts to 96% or more.