Dewatered Sludge Is Still Mostly Water: Choosing Equipment by Cake Solids Rather Than by Brochure

The water that will not leave

Sludge produced by wastewater treatment has a very high water content, and the way water binds to sludge particles sets a hard limit on what mechanics can achieve. Organic-matter content, ash ratio and especially flocculant dosage strongly affect the final solids figure. Mechanical dewatering generally yields a solids content of 20%–30%, and the product is called a sludge cake. It still has high moisture and fluid properties, so disposal remains difficult and costly and further volume reduction is necessary. Apart from natural air-drying, only the input of heat to drive evaporation achieves volume reduction at scale. That treatment is thermal drying.

Know which sludge you have before you buy anything

Six categories cover most of what arrives at a plant.

• Excess activated sludge from the secondary settling tank of domestic sewage plants: hydrophilic, fine-grained organic sludge, with poor compressibility and poor dewaterability.
• 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.
• Physicochemical and biochemical mixed sludge from the thickening tank of industrial wastewater treatment, covering paper mills, dyeing mills, garment washing, petrochemical plants, organic chemical plants, meat plants and breweries: medium-fine-grained mixed sludge, better dewaterability where fibre is present, otherwise average.
• Physicochemical fine-grained sludge from physical or chemical treatment of industrial wastewater, such as electroplating and circuit-board plants: fine-grained inorganic sludge, average compressibility and dewaterability.
• Physicochemical precipitated medium-grained sludge from industrial wastewater, including steel-plant desulfurization dust sludge, caustic soda brine mud, alumina red mud, ceramic-plant sludge, CRT-plant sludge and lime-neutralization sludge: medium-grained hydrophobic inorganic sludge, good compressibility and dewaterability.
• Physicochemical precipitated coarse-grained sludge from industrial wastewater, such as coal-washing tailings and glass-plant quartz residue: coarse-grained hydrophobic inorganic sludge, very good compressibility and dewaterability.

The first category generates most of the complaints. Poor compressibility is a property of the sludge, not a defect of the machine, and no vendor selection exercise changes it.

Four stages, four outcomes

A typical sludge treatment process runs thickening for initial volume reduction, digestion to decompose organic matter, dewatering for further volume reduction, and disposal as final disposition. Filtrate or supernatant from every stage still contains much pollution and should return to the wastewater treatment system. Done properly, the line achieves four transformations: volume reduction, so the sludge drops to a fraction of its original volume and turns from liquid to solid, easy to transport and dispose; stabilisation, so the easily putrefiable fraction is decomposed, odour is greatly reduced and handling improves; harmlessness, since primary sludge carries pathogens, parasite eggs and viruses and digestion kills most ascarid eggs, pathogens and viruses; and resource recovery, where the disposal route allows it.

Common simplified flows are sludge to thickening to drying bed to disposal; sludge to drying bed to disposal; sludge to thickening to dewatering to sanitary landfill; sludge to thickening to digestion to agricultural use; sludge to concentration to agricultural use; and sludge to concentration to dewatering to incineration, where digestion is often omitted so that calorific value is preserved.

Drying beds and the granulation route

A sludge drying bed is a flat area enclosed and divided by earth embankments, with a thin layer of gravel and sand plus underdrains laid where soil permeability is poor. Infiltration and evaporation together reduce the moisture of sludge placed on the bed. Infiltration takes about 2–3 days and lowers moisture to about 85%; evaporation then dominates, reaching about 75% after several weeks. Performance depends entirely on local rainfall, evaporation, temperature and humidity, so drying beds suit dry, low-rainfall, sandy-soil areas.

The in-water granulation dehydrator is a newer 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 enters the granulation section, where under its own weight it is flocculated, compressed and rolled into mud balls; balls and water then enter the dewatering section, where water drains through circumferential slits; in the compaction section the balls are compressed further under their own weight and discharged as dense, large mud balls. The granulator is simple, wear-resistant, low in power use and easy to maintain, and the mud balls usually come out at about 70% moisture.

Mechanical dewatering: filtration against centrifugation

Sludge is usually pre-treated to improve dewaterability, most commonly by dosing inorganic salts or polymer coagulants, though washing and heat treatment are also used. Filtration passes wet sludge through a filter layer of porous material such as filter cloth or wire mesh, letting water through as filtrate while retaining the dewatered sludge as cake. Centrifugation separates sludge and water by the different centrifugal tendencies of solid and liquid under a density difference.

Vacuum filters feed and discharge continuously and run steadily, but need many auxiliaries. Plate-frame filter presses, common in the chemical industry, have strong filtration driving force and low cake moisture but intermittent feed and discharge and lower productivity; manually operated units are labour-heavy and mostly automated now. Belt filters are the newer family, built in various designs around gravity, pressure, capillary and granulation principles, all with a rotating belt that conveys and dewaters sludge. Centrifugal dewatering usually means a horizontal high-speed settling centrifuge with inner and outer drums, cylindrical at one end and conical at the other, running at about 3000 rpm or higher with a set speed difference; it produces continuously with automatic control, good hygiene and small footprint, but demands better pre-treatment. Expect cake moisture of 60%–80% from vacuum filtration, 45%–80% from plate-frame and 80%–85% from centrifugation. Mechanical dewatering is mainly applied to primary-settling and digested sludge.

Two machines, compared plainly

The plate-frame sludge dewaterer works in a closed state: sludge pumped in by a high-pressure pump is squeezed by the plates and water passes through the filter cloth. It is cheap, good at dewatering inorganic sludge and gives low cake moisture, but it clogs easily, needs a high-pressure pump, is unsuitable for oily sludge, and is hard to run continuously and automatically.

The belt sludge dewaterer uses a continuously moving filter belt under gravity and pressure. It suits many sludges and runs continuously, but cake moisture is relatively high and the belt needs careful washing.

Sludge as a resource

Sludge is a resource carrying much heat, with a calorific value of 10000–15000 kJ/kg on a dry sludge basis, higher than coal and coke. It is also rich in nitrogen, phosphorus and potassium, making it a high-value organic fertiliser. Routing it through digestion converts organic matter into biogas, capturing that heat and improving fertiliser value.

Common sludge-thickening processes are gravity thickening, centrifugal thickening and flotation thickening. Sludge digestion divides into anaerobic and aerobic digestion. Dewatering divides into natural drying and mechanical dewatering, most commonly belt-filter pressing and centrifugal dewatering. Disposal routes include agricultural and forestry use, sanitary landfill, incineration and building-material production.

One thing catches plants out: filtrate produced during sludge dewatering in a sewage plant contains high pollutant concentrations, drying-bed filtrate excepted, so it must be treated, generally together with the incoming wastewater.