Infrastructure, Networks & Equipment
Sludge Thickening Companies
Gravity, DAF, drum, and rotary thickener suppliers for upstream sludge concentration and digester feed optimization.
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Sludge Thickening Processes: Gravity, DAF, and Centrifuge Performance for WwTW
Sludge thickening increases the solids concentration of dilute sludge (0.5 to 2 percent DS) before anaerobic digestion, dewatering, or disposal, reducing digester volumes and improving downstream process efficiency. Primary sludge (raw, settled in primary clarifiers): DS typically 2 to 6 percent; thickens readily to 5 to 10 percent DS by gravity in a thickener at 1 to 2 days HRT and surface loading rate 15 to 30 kg DS/m2/day. Waste activated sludge (WAS, from biological treatment): DS typically 0.5 to 1.5 percent; bulking properties of biological floc make gravity thickening challenging; achievable DS 1.5 to 3 percent at very low loading rates. Mixed sludge (primary + WAS blend): intermediate thickening behaviour. Thickening technologies: gravity thickeners (circular, peripheral or centre drive, with or without picket fence agitator); gravity belt thickeners (GBTs); dissolved air flotation (DAF) thickeners; rotary drum thickeners (RDTs); centrifuge thickeners. Technology selection depends on sludge type, DS target, chemical conditioning, and space constraints.
Dissolved air flotation (DAF) thickening is highly effective for biological sludge (WAS) that settles poorly. DAF process: recycle stream (20 to 50 percent of feed flow) pressurised to 4 to 6 bar in a saturator vessel with air injection; dissolved air releases as microbubbles (20 to 100 micron) at atmospheric pressure in the flotation tank; microbubbles attach to sludge floc and float it to the surface as a floated sludge layer; overflow (thickened sludge, 3 to 6 percent DS for WAS); underflow (clarified effluent returned to head of works). DAF surface loading rate: 3 to 8 m3/m2/h at 3 to 6 percent recycle air-to-solids ratio. Polymer conditioning: cationic polyacrylamide (PAM) at 3 to 8 g/kg DS improves bubble attachment and floc integrity; increases DS by 1 to 2 percentage points. DAF thickened WAS: 3 to 6 percent DS vs 1.5 to 3 percent for gravity. Gravity belt thickeners (GBT): sludge conditioned with polymer (3 to 8 g/kg DS) applied to moving porous belt, gravity drainage and gentle squeezing achieves 3 to 6 percent DS for WAS; flow rate 10 to 40 m3/hour per meter belt width.
Centrifuge thickening provides the highest DS and smallest footprint. Decanter centrifuge (horizontal, solid bowl, continuous): G-force 1,500 to 3,000 x g; WAS thickened to 4 to 8 percent DS; feed rate 10 to 100 m3/hour; polymer conditioning 3 to 8 g/kg DS required; centrate clarity 200 to 2,000 mg/L SS (returned to inlet). High-speed disc stack centrifuges (Alfa Laval, GEA Westfalia): smaller footprint than decanter; effective for lower-volume applications. Rotary drum thickeners (RDT): polymer-conditioned sludge rotates in a drum with fine screen apertures; gravity drainage and gentle mixing; achieves 3 to 6 percent DS for WAS; low energy consumption; simple operation. Thickening before anaerobic digestion: increasing WAS DS from 1 percent to 4 percent reduces digester volume by 4-fold, dramatically improving economic viability of AD. UK WwTW commonly use GBT or DAF for WAS thickening with subsequent digestion; performance monitored against design DS targets in EA permits. Chemical conditioning optimisation: jar testing (varying polymer dose and type) should be conducted regularly as sludge characteristics vary seasonally with biological process conditions.
Frequently Asked Questions
What is the difference between sludge thickening and dewatering?
Sludge thickening and dewatering are both volume reduction processes but operate at different DS levels and use different mechanisms: Thickening: increases DS from 0.5 to 2 percent (liquid sludge) to 3 to 8 percent (thickened sludge, still pumpable as a viscous liquid); achieved by gravity settling, flotation, or gentle mechanical separation (GBT, RDT, centrifuge); energy consumption low (typically 5 to 15 kWh/tonne feed); output remains liquid/semi-liquid and is pumped. Purpose: reduce volume before digestion (reduces digester size and heating requirement), improve digester mixing efficiency. Dewatering: increases DS from 3 to 8 percent (thickened sludge) to 15 to 40 percent (cake, semi-solid); achieved by high-force mechanical processes (centrifuge at 2,000 to 3,000 G, belt press at 5 to 10 bar, filter press at 7 to 15 bar); energy consumption higher (15 to 40 kWh/tonne feed); output is a handleable cake suitable for conveyor, skip, or truck transport. Sequence: liquid sludge - thickening - digestion - dewatering - cake disposal. Some small works omit thickening if digester sizing is sufficient to handle dilute feed.
How does DAF thickening work?
Dissolved Air Flotation (DAF) thickening uses microscopic air bubbles to float sludge solids to the surface for collection. Operating principle: (1) Pressurisation: a recycle stream (20 to 50 percent of feed) is pressurised to 4 to 6 bar in a saturation vessel; compressed air dissolves into the water under pressure (Henry's Law: solubility increases with pressure); (2) Release: recycle stream re-enters the flotation tank at atmospheric pressure via a pressure release valve; dissolved air comes out of solution as fine bubbles (20 to 100 micron) - the driving force for flotation; (3) Flotation: bubbles attach to and are entrapped within sludge floc; buoyant bubble-floc aggregates rise to the surface; (4) Collection: rotating skimmer collects thickened floated sludge (float layer 150 to 400 mm deep); thickened sludge removed by overflow weir or pump; (5) Effluent: clarified underflow (low SS) returned to inlet works. Polymer conditioning (cationic PAM, 3 to 8 g/kg DS): critical for WAS - forms larger, stronger floc that entraps bubbles more effectively; increases thickened DS by 1 to 2 percentage points above unpolymerised. DAF achieves 3 to 6 percent DS for WAS vs 1.5 to 3 percent for gravity thickening.
What polymer dose is needed for sludge thickening?
Polymer conditioning dose for sludge thickening depends on sludge type and thickening technology: Gravity belt thickener (GBT): WAS requires 4 to 10 g/kg DS cationic PAM (higher dose than centrifuge due to lower shear forces available for polymer mixing); primary sludge requires 1 to 3 g/kg DS. DAF thickening: 3 to 8 g/kg DS cationic PAM for WAS; 1 to 3 g/kg DS for primary; optimum charge density depends on sludge surface charge (Zeta potential: ideally drive to near-zero, typically -5 to +5 mV). Centrifuge thickening: 2 to 6 g/kg DS for WAS. Polymer selection: linear cationic PAM; charge density (10 to 50 percent active groups) and molecular weight (5 to 15 million Dalton) matched to sludge by jar testing. Over-dosing polymer increases cost (polymer typically GBP 2 to 5 per kg) and may cause re-dispersion (positive Zeta potential) or carry-over of polymer into thickened sludge affecting downstream digestion. Under-dosing causes poor thickening performance (low DS, high centrate SS). Jar testing protocol: OECD 211 equivalent; dose range 2 to 20 g/kg DS tested in 1-litre cylinders; measure drainage rate and DS after 30 minutes settling to identify optimum dose.
What DS can be achieved by sludge thickening?
Achievable DS by thickening technology and sludge type: Primary sludge: gravity thickener 5 to 10 percent DS; GBT 6 to 10 percent DS; centrifuge 8 to 12 percent DS. Waste activated sludge (WAS, biological): gravity thickener 1.5 to 3 percent DS (poor settling); DAF thickener 3 to 6 percent DS with polymer; GBT 3 to 6 percent DS with polymer; RDT 3 to 5 percent DS; decanter centrifuge 4 to 8 percent DS. Mixed primary + WAS (50:50 blend by volume): gravity thickener 3 to 5 percent DS; GBT 5 to 7 percent DS; centrifuge 6 to 10 percent DS. Digested sludge (after AD): gravity thickener 2 to 4 percent DS; centrifuge thickening before dewatering 4 to 7 percent DS. Design targets for UK WwTW: thickened sludge to anaerobic digester typically 4 to 6 percent DS (optimum for digester performance and mixing); higher DS reduces heat requirement for pasteurisation (55 degrees C, 1 hour per BS EN 14701); lower DS increases digester volume requirements. Actual DS performance varies with sludge age, temperature, biological process type (extended aeration WAS thickens better than short SRT WAS), and polymer selection.
A 200,000 PE wastewater treatment works in the North East of England was running its mesophilic anaerobic digesters at 3.2% DS feed concentration (limited by WAS thickening performance from two ageing gravity thickeners). The dilute feed was causing digester mixing problems, reducing volatile solids destruction to 38%, and limiting biogas CHP output to 62% of design capacity.
The operator replaced the two gravity thickeners with a single dissolved air flotation (DAF) thickener (surface area 45 m2, recycle ratio 35%), fitted with an automated polymer dosing system using a cationic polyacrylamide selected through jar testing on site sludge. DAF operating pressure was set at 5 bar. The system was designed to achieve 5% DS thickened WAS, blended with primary sludge to feed the digesters at 5.5% DS.
DAF thickened WAS consistently achieved 4.8% DS. Blended digester feed rose to 5.6% DS. Volatile solids destruction improved from 38% to 52%. Biogas production increased by 34%, lifting CHP electrical output from 620 kW to 830 kW and reducing grid electricity import by GBP 290,000 per year. Polymer consumption was 6.8 g/kg DS, within the design budget.
Questions to Ask Shortlisted Providers
- 1
What is the current WAS DS concentration and what target thickened DS is required before digestion or dewatering?
The gap between current and target DS defines the thickening duty; a target of 5 to 6% DS typically requires DAF or centrifuge thickening rather than gravity-only.
- 2
What is the WAS settleability (Sludge Volume Index, SVI) and does the plant experience bulking?
High SVI sludge (above 150 mL/g) thickens poorly by gravity; DAF or centrifuge thickening is essential for filamentous bulking sludge; SVI should be characterised across seasonal variation.
- 3
What polymer has been trialled and at what dose, and has jar testing been conducted on current sludge?
Polymer demand varies with sludge age, temperature, and biological process; jar testing on actual sludge identifies the optimum polymer type (charge density, molecular weight) and dose, avoiding over-specification on supply-only contracts.
- 4
What are the space constraints for the new thickening equipment and is there crane access for maintenance?
DAF units require horizontal footprint; centrifuges have smaller footprint but require lifting gantry access; GBT units require belt wash water drainage and odour enclosure.
- 5
What is the thickener reject or centrate quality and what is the return load impact on inlet works?
Thickener centrate or DAF subnatant returns high ammonia and SS loads to the inlet; quantifying this return load is critical for inlet works capacity assessment and EA permit compliance.
What Drives Cost in This Category
DAF thickener units for 50,000 to 200,000 PE WwTW cost GBP 250,000 to 750,000 supply and install; civil works (concrete base, pipework, odour ventilation) add GBP 80,000 to 200,000.
Cationic PAM polymer costs GBP 2 to 6 per kg active; at 6 g/kg DS and 5 tonnes DS/day throughput, polymer cost is GBP 60 to 180 per day; optimised jar-tested dose selection typically saves 15 to 25% over generic specification.
GBT units cost GBP 120,000 to 350,000 per unit installed; lower capital than DAF but require more belt wash water (2 to 4 m3/m/hour) and manual belt inspection; suitable for primary or mixed sludge where DAF is not justified.
Gravity thickening uses 0.5 to 2 kWh/tonne feed; DAF uses 10 to 20 kWh/tonne feed; centrifuge uses 15 to 30 kWh/tonne DS; however, the energy value of additional biogas generated from improved digester feed DS typically exceeds the additional thickening energy cost.
Key Regulations & Standards
EA Environmental Permits for WwTW specify minimum VS destruction targets (typically 38 to 50% for agricultural biosolids) and pathogen reduction standards (BS EN 14701 pasteurisation for ETS classification); improved thickening directly supports VS destruction compliance.
Conventional anaerobic digestion with mesophilic temperature (35 degrees C, HRT 15+ days) qualifies as Conventional Treatment under the Safe Sludge Matrix; enhanced digester performance from improved thickening does not require reclassification but supports the agricultural application route.
Liquid polymer emulsions are classified as irritants and require COSHH assessment, MSDS review, and PPE (nitrile gloves, safety glasses) for operators; spill containment bunding required in polymer storage areas per Environmental Permitting Regulations 2016.
WIA 1991 requires water companies to dispose of sludge without causing environmental harm; Ofwat AMP8 totex framework rewards improved biogas yield and energy self-sufficiency; thickening investments that increase CHP output are eligible for totex allowance within the AMP8 business plan.