What Is Sludge Dewatering?
Sludge produced by wastewater treatment has a high water content. Because of the way water binds to sludge particles, mechanical dewatering has certain limits; the organic-matter content, ash ratio and especially the flocculant dosage strongly affect the final solids content. Generally, mechanical dewatering yields a solids content of 20%–30%, and the resulting sludge is called a sludge cake. The cake still has a high moisture content and fluid properties, so its disposal remains difficult and costly, and further volume reduction is necessary. Apart from natural air-drying, only input of heat to drive evaporation can achieve large-scale volume reduction; thermal drying is the heat-based treatment.
Sludge classification
1. Excess activated sludge from the secondary settling tank of domestic sewage plants: hydrophilic, fine-grained organic sludge, with poor compressibility and poor dewaterability.
2. Physicochemical sludge from the settling or thickening tank of a water-treatment plant: medium-fine-grained organic-inorganic mixed sludge, with average compressibility and dewaterability.
3. Physicochemical and biochemical mixed sludge from the thickening tank of industrial wastewater treatment (paper mills, dyeing mills, garment washing, petrochemical plants, organic chemical plants, meat plants, breweries, etc.): medium-fine-grained mixed sludge; where fiber is present dewaterability is better, otherwise average.
4. Physicochemical fine-grained sludge from physical/chemical treatment of industrial wastewater (electroplating, circuit-board plants, etc.): fine-grained inorganic sludge, average compressibility and dewaterability.
5. Physicochemical precipitated medium-grained sludge from industrial wastewater (steel-plant desulfurization dust sludge, caustic soda brine mud, alumina red mud, ceramic-plant sludge, CRT-plant sludge, lime-neutralization sludge, etc.): medium-grained hydrophobic inorganic sludge, good compressibility and dewaterability.
6. Physicochemical precipitated coarse-grained sludge from industrial wastewater (coal-washing tailings, glass-plant quartz residue, etc.): coarse-grained hydrophobic inorganic sludge, very good compressibility and dewaterability.
Resource recovery
Sludge is a resource containing much heat, with a calorific value of 10000–15000 kJ/kg (dry sludge), higher than coal and coke. It is also rich in nitrogen, phosphorus and potassium, an organic fertilizer of high value. Through the digestion stage of the process, organic matter is converted into biogas, utilizing its heat and further improving fertilizer value. Common sludge-thickening processes are gravity thickening, centrifugal thickening and flotation thickening. Sludge digestion is divided into anaerobic and aerobic digestion. Sludge dewatering is divided into natural drying and mechanical dewatering; common mechanical dewatering processes are belt-filter pressing and centrifugal dewatering. Sludge disposal routes include agricultural/forestry use, sanitary landfill, incineration and building-material production.
Drying beds and granulation dewatering
The main structure of a sludge drying bed is a flat area enclosed and divided by earth embankments; if the soil permeability is poor, a thin layer of gravel and sand with underdrains may be laid. Reliance on infiltration and evaporation reduces the moisture of the sludge placed on the bed. Infiltration takes about 2–3 days, lowering moisture to about 85%; thereafter evaporation dominates, reaching about 75% after several weeks. Drying-bed performance depends on local rainfall, evaporation, temperature and humidity, and is generally suitable for dry, low-rainfall, sandy-soil areas.
The in-water granulation dehydrator is a new device: a horizontal cylinder of steel plate divided into a granulation section, a dewatering section and a compaction section, rotating slowly about a horizontal axis. Sludge dosed with polymer coagulant first enters the granulation section, where under its own weight it is flocculated and compressed, rolled into mud balls; the balls and water then enter the dewatering section, where water drains through circumferential slits. Finally in the compaction section the balls are further compressed under their own weight, forming dense, large mud balls discharged from the cylinder. The granulator is simple, wear-resistant, low in power use and easy to maintain; the mud balls usually have a moisture content of about 70%.
The filtrate produced during sludge dewatering in a sewage plant contains high pollutant concentrations except for drying-bed filtrate; it must therefore be treated, generally together with the incoming wastewater.
Mechanical dewatering methods
Sludge is usually pre-treated to improve dewaterability before dewatering; the most common pre-treatment is dosing with inorganic salts or polymer coagulants, and there are also washing and heat-treatment methods. Mechanical dewatering includes filtration and centrifugation. Filtration passes wet sludge through a filter layer (porous material such as filter cloth or wire mesh), letting water (filtrate) pass through while retaining the dewatered sludge (cake). Centrifugation uses the different centrifugal tendencies of solid and liquid due to density difference to separate sludge and water. Filtration equipment includes vacuum filters, plate-frame filter presses and belt filters. Vacuum filters feed and discharge continuously with stable operation but many auxiliaries. Plate-frame filter presses, common in the chemical industry, have strong filtration driving force and low cake moisture but intermittent feed/discharge and lower productivity; manually operated ones have heavy labor and are mostly automated. Belt filters are new, with various designs based on different dewatering principles (gravity, pressure, capillary, granulation), all with a rotating belt that conveys and dewaters sludge. Centrifugal dewatering commonly uses a horizontal high-speed settling centrifuge of inner and outer drums, one end cylindrical and the other conical, at about 3000 rpm or higher, with a certain speed difference; it produces continuously with automatic control, good hygiene and small footprint but higher pre-treatment requirements. Mechanical dewatering is mainly used for primary-settling and digested sludge. Cake moisture is generally 60%–80% for vacuum filtration, 45%–80% for plate-frame, and 80%–85% for centrifugation.
Typical sludge treatment process
A typical sludge treatment process includes four stages: (1) thickening — initial volume reduction; (2) digestion — decomposition of organic matter; (3) dewatering — further volume reduction; (4) disposal — final disposition. The filtrate or supernatant from each stage still contains much pollution and should return to the wastewater treatment system. This process achieves the "four transformations":
(1) Volume reduction: after treatment the sludge volume drops to a fraction of the original and changes from liquid to solid, easy to transport and dispose.
(2) Stabilization: high organic content easily putrefies and smells; after digestion the easily putrefiable part is decomposed, odor is greatly reduced, and transport/disposal are easier.
(3) Harmlessization: especially primary sludge contains many pathogens, parasite eggs and viruses that can spread disease; digestion kills most ascarid eggs, pathogens and viruses, greatly improving hygiene.
Simplified flows: Sludge → thickening → drying bed → disposal; or Sludge → drying bed → disposal; Sludge → thickening → dewatering → sanitary landfill; Sludge → thickening → digestion → agricultural use; Sludge → concentration → agricultural use; Sludge → concentration → dewatering → incineration (incineration often omits digestion to preserve calorific value).
Common dewatering equipment
1. Plate-frame sludge dewaterer: working principle — in a closed state, sludge pumped in by a high-pressure pump is squeezed by the plates, and water passes through the filter cloth to achieve dewatering. Advantages: low price, good at dewatering inorganic sludge, low cake moisture. Disadvantages: easily clogged, needs a high-pressure pump, unsuitable for oily sludge, hard to run continuously and automatically.
2. Belt sludge dewaterer: uses a continuously moving filter belt under gravity and pressure to dewater; suitable for various sludges, continuous operation, but cake moisture is relatively high and it requires careful belt washing.
