Mechanical Dewatering

Dewatering removes water from sludge (residual sludge from wastewater treatment plants such as activated sludge systems, or faecal sludge from on-site sanitation such as septic tanks or pit latrines) making it easier to handle and less expensive to transport, to compost (see also small or large scale composting), to incinerate (see also small or large scale incineration) or to dispose of in a landfill. Dewatering can be done either with the help of presses or centrifuges (mechanical dewatering), filtration (e.g. in filter bags) or physical (and biological) processes in drying beds. Mechanical dewatering and filtration often require the use of chemicals (coagulants and flocculants) to enhance the process.

Belt Filter Press

Modern belt presses are based on a combination of chemical conditioning, gravity drainage and mechanical pressure in a continuous feed system to dewater sludge. The sludge is squeezed between tensioned serpentine belts and a series of decreasing diameter rollers (to increase the pressure) to remove moisture and create a dewatered sludge cake (JOHNSONSCREENS 2011).

Chamber Filter Press

Chamber filter presses use filters to separate the liquid (filtrate) from the sludge and collect the solids (filter cake) in chambers. A number of such chambers for the collection of the solids are arranged in a plate pack. The filter cloth is supported by the plates of the plate pack. The sludge (optionally mixed with coagulant) is pumped into the chambers and against the filter media. A moving plate (the tail plate) clamps the filter plate pack. The filtrate passes through the filter cloth and exits through the filtrate ports of the filter plate. The filter cake collects on the surface of the filter medium within the chambers until the chamber is full. The cake may be washed or blown dry if necessary. There are manual, semi-automatic and automatic chamber filter presses available.

Centrifuge

Adapted from EPA (2000)

Centrifuges operate as continuous feed units, which remove solids by a scroll conveyor and discharge liquid over the weir. The bowl is conical-shaped which helps lift solids out of the liquid allowing them to dry on an inclined surface before being discharged. The picture shows a typical centrifuge thickening and dewatering system.

These systems are mostly high-tech and require expert design, which increases costs. As the techniques are usually sophisticated they are rarely cost-efficient for smaller systems to be implemented on community level (HEEB et al. 2008). The most cost effective method of sludge dewatering depends on many factors including plant size, the cost of further processing, end-use (e.g. if the thermal heat can be recovered by incinerating the dried sludge), disposal, odour control requirements, and space limitations (EPA 2000).

These technical systems require a high degree of operator supervision, and operator training. The mechanical systems such as filters, belts, tensioning systems or bearings have to be maintained correctly to guarantee proper functioning. The performance of primary treatment systems prior to mechanical stage is also a critical determinant of performance. Primary treatment commonly includes the removal of gross solids, and slurry thickening. Routine use of flocculants is essential, normally selected at installation using advice and the results of on-site testing from polymer chemical suppliers (FSA 2002).

The liquid part needs to be treated: it is either circulated back into the wastewater treatment plant from where the sludge comes from (e.g. activated sludge or treated in vertical flow or horizontal flow constructed wetlands). The solid part has to be burnt in a small or large scale incineration plant or discharged in a landfill; it may also be a composted (see also small or large scale composting facility).

This process does not serve the treatment of wastewater; it only separates solids from liquids. Both solid and liquid parts still contain pathogens and pollutants (HEEB et al. 2008) and have to be treated (see Operation and Maintenance).