INTRODUCTION
Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.
Sludge treatment is focused on reducing sludge weight and volume to reduce disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.
Sludge dewatering systems can be custom engineered and integrated into your process to remove solid waste, extracting the water from sludge. Choosing the correct sludge dewatering system that is right for you and your process is vital for maximum efficiency and to not add operating costs.
Thickening
Thickening is often the first step in a sludge treatment process. Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates. Primary sludge may be thickened to about 8 or 10 percent solids, while secondary sludge may be thickened to about 4 percent solids. Thickeners often resemble a clarifier with the addition of a stirring mechanism. Thickened sludge with less than ten percent solids may receive additional sludge treatment while liquid thickener overflow is returned to the sewage treatment process.
Dewatering
- Belt filter – Schematic of a belt filter press to dewater sewage sludge. Filtrate is extracted initially by gravity, then by squeezing the cloth through rollers. Before sludge enters the press it is chemically conditioned for dewatering with an emulsion polymer flocculant that helps form stronger flocs. After chemical conditioning, a transfer pump drops the sludge onto a preliminary dewatering belt where free water molecules separate by gravity and fall into a collection trough.
As sludge is conveyed along the belt, ploughs roll it around to help water drain out. Before dropping down to the next stage, guide plates position the sludge towards the middle of the belt and ensure nothing is squeezed out the side of the filter. A second gravity thickener repeats the process before sludge is fed into a pressing zone. Upon entering the pressing zone, sludge is sandwiched and squeezed between two belt filters which slowly convey over and under rollers that force excess water out of the sludge and through the filter mesh.
Finally, the pressed sludge is scraped off the belt and collected in a bin. All filtrate and wash water is captured and transferred back to the front of the wastewater system for re-processing.
- Centrifuge – A centrifuge is a device, which employs a high rotational speed to separate components of different densities. This becomes relevant in the majority of industrial jobs where solids, liquids and gases are merged into a single mixture and the separation of these different phases is necessary. A decanter centrifuge separates solid materials from liquids in slurry and therefore plays an important role in wastewater treatment, chemical, oil and food processing industries. There are several factors that affect the performance of a decanter centrifuge and some design heuristics to be followed which are dependent upon given applications. The operating principle of a decanter centrifuge is based on separation via buoyancy. Naturally, a component with a higher density would fall to the bottom of a mixture, while the less dense component would be suspended above it. A decanter centrifuge increases the rate of settling through the use of continuous rotation, producing a force equivalent to between 1000 and 4000 G’s. This reduces the settling time of the components by a large magnitude, whereby mixtures previously having to take hours to settle can be settled in a matter of seconds using a decanter centrifuge. This form of separation enables more rapid and controllable results.
- Plate Filter Press – This pressing technique is the most widespread despite its intermittent operation and its high investment cost.
Description
A filter comprises a set of vertical, juxtaposed recessed plates, presses against each other by hydraulic jacks at one end of the set. The pressure applied to the joint face of each filtering plate must withstand the chamber internal pressure developed by the sludge pumping system.
This vertical plate layout forms watertight filtration chambers allowing easy mechanisation for the discharge of cakes. Filter clothes finely or tightly meshed are applied to the two growled surfaces in these plates.
Orifices feed the sludge to be filtered under pressure in the filtration chamber. They are usually placed in the center of the plates allowing a proper distribution of flow, right pressure and better drainage of sludge within the chamber. Solids sludge gradually accumulates in the filtration chamber until the final compacted cake is formed. The filtrate is collected at the back of the filtration support and carried away by internal ducts.
Filtration cycle
The filter press is an intermittent dewatering process. Each press operation includes the following steps:
1- Closing of the press: as the filter is completely empty, the moving head activated by the jacks calmps the plates. Closing pressure is self-regulated through filtration.
2- Filling: During this short phase chamber are filled with sludge for filtration. Filling time depends on the flow of the feed pump. For sludge having good filterability it is best to fill the filter very quickly so as to avoid the formation of a cake in the first chamber before the last ones have been filled.
3- Filtration: Once the chamber has been filled continuous arrival of sludge to be dewatered provokes a rise in pressure due to the formation of an increasingly thick layer of filter sludge on the cloths. This filtration phase can be stopped manually, by a timer or more conveniently by a filtrate flow indicator which issues a stop alarm when the end of filtration rate has been reached. When the filtration pump has stopped, the filtrate circuits and central duct, which is still filled with liquid sludge, are purged by compressed air.
4- Filter opening: The moving head is drawn back to disengage the first filtration chamber. The cake falls has a result of his own weight. A mechanised system pulls out the plates one by one. The speed of plate separation can be adjusted to account to the cake texture.
5- Washing: Washing of clothes should be carried out every 15-30 processing operations. For mid- or large units this take place on press using water sprayers at very high pressure (80-100 bar). Washing is synchronized with separation of plates.
Filtration capacity
The production capacity of a filter press is somewhere between 1.5 and 10 kg of solid per m2 of filtering surface. For the filter press model the chamber volume and the filtering surface depend on the number of plates in the filer.
In practical terms pressing times are less then four hours.
Filtration time depends on:
- cake thickness
- sludge concentration
- specific resistance
- compressibility coefficient.
One of the advantages of the filer press is that it can accept sludge with average filterability. It is always advantageous to optimally thicken sludge before filter press operations. Although sludge presenting a high filterability enables better production capacities, a filter press still accepts sludge with low conditioning precision. This tolerance means that the device offers greater overall operational safety.
Performance
The filter press is suitable for almost all types of sludge:
- Hydrophilic organic sludge: inorganic conditioning is often recommended to enable satisfactory cake release due to minimal adherence to filter cloth.
- Hydrophilic inorganic sludge: the filer press generally requires the addition of lime only.
- Hydrophobic inorganic sludge: it is very dense and ideal for the filter press. It is dewatered without any preliminary conditioning.
- Oily sludge: the filter press can be used to treat sludge containing light oils, the presence of grease can sometimes impair the smooth running of the filter; clothes have to be degreased at frequent intervals.
