1. Introduction
The production of sludge is an inescapable aspect of the sewage treatment process. The places where sludge is produced in a typical mechanical and biological sewage treatment plant are the primary and secondary sedimentation tanks. To keep things as simple as possible, the sludge produced in the primary sedimentation tank comprises suspension and compounds adsorbed on its surface, while the sludge from the secondary sedimentation tank may be described as excess sludge, produced as a result of growth in the biomass and sediment of the biologically non-degradable suspension that filtered through from the preliminary tank.
The mass and volume of sludge is not constant and unchanging over time. The relationship of mass and volume in sludge stabilisation and dewatering processes is presented in the chart below:
The sludge produced in the treatment process should be processed into harmless end products and removed from the treatment plant.
2. Thickening of sludge
Sludge comprises particulates and water. The more water in the sludge, the more liquid and hydrated the sludge. Water contained in sludge comes in three forms: gravitational (intermolecular) water, capillary water and chemically bound water. Only gravitational water is removed from the sludge in the thickening process. Gravitational water is water that separates spontaneously from the sludge, e.g. during settling or filtering off. Gravitational water dominates in hydrated sludge – from 80% to 99%.
Sludge thickening may involve:
(a) sludge particles voluntarily settling due to the force of gravity;
(b) floating sludge particles are bound with a floating factor (most often air) and raised to the surface of the thickener;
(c) mechanical thickening, when gravitational water is separated from the sludge in a centrifuge or filtration process.
3. Dewatering of sludge
Dewatering is the process of removing water from sludge in order to reduce its volume. Gravitational water is removed from the sludge in the thickening process, while capillary water is removed in a mechanical or natural dewatering process. Removing of capillary water usually happenms with dry contents in beetwen 50% and 80%. Bound (adsorption) water is removed during a drying process. Bound water usually happens with dry contents between 0.1% and 50%. Dewatering is used for both raw and stabilised sludge. The basic sludge dewatering processes are presented in the diagram below:
Dewatering enables a significant reduction in the amount of sludge for further processing, and in addition after the dewatering its consistency changes from liquid to that of loose material, thereby significantly improving the transport options.
Above: sludge after centrifugal dewatering
When mechanically dewatering sludge the sludge is conditioned beforehand in order to improve the dewatering result by reducing its resistivity. Currently polymers are mainly used for the conditioning. The polymers should be selected individually for every sludge. It has to be stressed that, just as there is no such thing as typical sludge, there are no typical polymer doses.
4. The economics
A complete sludge dewatering installation comprises numerous pieces of equipment, the operation and maintenance of which requires specialist technical knowledge. This installation includes mechanical equipment for dewatering the sludge (centrifuges, presses), a polymer solution preparation and dosing unit, a sludge feed system, a system for collecting the dewatered sludge and an extensive control and measurement system.
The costs of constructing and equipping a sludge dewatering station are significant. In addition there are the costs related to the equipment’s depreciation, its operation, servicing and maintenance, repairs and overhauls. As such the question arises about how rational it would be to construct one’s own stationary sludge dewatering station.
The economic calculation shows that for treatment plants for which the quantity and hydration of the sludge produced is below a certain value, a financially viable alternative to constructing one’s own dewatering station is to use the services of a company operating mobile sludge dewatering stations.
An example cost simulation carried out for an urban treatment plant with average daily sewage inflow of 7000 m3/d showed that the cost of having the sludge dewatering handled by an external company amounted annually to approximately 10% of the costs of building one’s own installation. A complete calculation, taking into account the capital for the investment as well as the on-going running costs, repairs, parts, specialist servicing and personnel costs, and also assuming that the installation’s depreciation would cover 15 years, shows that the cost of having sludge dewatered by an outside company amount annually to barely 4% of the investment costs. This means that for the assumed period of depreciation the service costs for sludge dewatering would amount to barely 60% of the costs of building and running one’s own dewatering station. The above simulation is based on the following assumptions related to the dewatering station’s requirements: purchase of new equipment by reputable companies, construction from scratch of the building for the dewatering station and the station’s installation, own equipment operating for one shift for five days each week, operated by a single operator on a shift and serviced by the producer’s service department.
Comparison of annual maintenance costs for own Sludge Dewatering Station to the costs for outsourcing the dewatering to a service company
5. Mobile Sludge Dewatering Station – comparison of press and centrifuge
From the client’s point of view, the technical differences between stations based on a press and those constructed with a centrifuge are of secondary importance. However, the performance and running costs are important.
Although the sludge dewatering result is similar for the two types of equipment, flocculant consumption – and therefore the total cost of the service – tends to be higher in the case of presses. Likewise in regard to the consumption of process water, as presses require periodical rinsing whilst centrifuges only consume water for preparation of the flocculant solution. In addition, as presses are significantly larger in size, they also require appropriate site preparation.
To summarise, one could say that the technical aspects indicate lower running costs for centrifuges than for presses.
6. Other applications for a mobile dewatering station
To finish off it is worth mentioning that mobile sludge dewatering stations can prove extremely helpful during the investment decision process related to stationary equipment. The profile of the sludge depends both on the technological process and the nature of its inflow. These values differ between all treatment plants, and as such choosing equipment according to a typical sludge profile may involve a certain degree of risk, as there is in fact no such thing as typical sludge.
Above: mobile tests at the company Drobimex-Heinz Sp. z o.o. in Szczecin