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Turbidity Removal Companies
Clarification and filtration providers for turbidity, coag/floc, DAF, sand, multimedia, and membrane processes.
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Turbidity Removal in Drinking Water: Coagulation, Flocculation, and Membrane Filtration
Turbidity in water is caused by suspended particles including clay, silt, algae, and organic colloids that scatter light (measured in NTU, Nephelometric Turbidity Units, per ISO 7027). Turbidity is both an aesthetic concern and a pathogen surrogacy indicator: high turbidity protects bacteria and protozoa (Cryptosporidium, Giardia) from UV and chlorine disinfection. WHO Guideline: turbidity less than 1 NTU for effective disinfection; less than 0.1 NTU for optimum UV and chlorine efficacy. UK DWI Standard: turbidity at consumer tap less than 4 NTU (mandatory, Water Supply (Water Quality) Regulations 2016); WwTW effluent turbidity less than 0.5 NTU for Cryptosporidium risk management (Cryptosporidium (Additional Measures) Direction 2022 following DWI/EA risk assessment). US EPA Turbidity MCL: less than 1 NTU in 95 percent of monthly samples from conventional treatment; less than 0.3 NTU in 95 percent of samples from filtered surface water systems (SWTR, Surface Water Treatment Rule; ESWTR, Enhanced SWTR). Source water turbidity drives treatment intensity: rivers typically 2 to 200 NTU routine, 100 to 1,000+ NTU in storm events; impounded reservoirs 0.5 to 20 NTU; groundwater typically less than 1 NTU.
Coagulation-flocculation-sedimentation is the primary turbidity removal sequence at surface water treatment works. Coagulation: destabilises colloidal particles by adding trivalent metal salt coagulants (aluminium sulphate Al2(SO4)3, ferric sulphate Fe2(SO4)3, ferric chloride FeCl3, or polyaluminium chloride PACl) at dose 5 to 50 mg/L as metal. Coagulation mechanisms: charge neutralisation (reducing Zeta potential from -30 to -60 mV to near-zero, -5 to +5 mV, allowing particle aggregation); sweep flocculation (at higher doses, metal hydroxide precipitate enmeshes particles). Rapid mixing: G (velocity gradient) 300 to 1,000 s-1 for 10 to 60 seconds. Flocculation: slow mixing (G 20 to 80 s-1, 10 to 30 minutes) grows floc from 1 to 5 micron to 50 to 500 micron settable particles; GT product (camp number) 10 to the 4 to 10 to the 5. Sedimentation: conventional clarifiers achieve turbidity 2 to 10 NTU; dissolved air flotation (DAF) achieves less than 2 NTU for algae-rich or low-turbidity source water. Post-filtration: dual-media rapid gravity filter (anthracite 1.0 to 1.4 mm / sand 0.5 to 0.85 mm, depth 1.0 to 1.5 m) or pressure filter achieves effluent turbidity less than 0.2 NTU. Combined coagulation-DAF-filtration achieves less than 0.1 NTU for Cryptosporidium control.
Membrane filtration for turbidity removal: low-pressure membranes (microfiltration MF, 0.1 to 0.2 micron, and ultrafiltration UF, 0.01 to 0.1 micron) provide absolute barrier to turbidity-causing particles and Cryptosporidium oocysts (4 to 6 micron). MF/UF turbidity removal: effluent consistently less than 0.1 NTU regardless of source water turbidity; Cryptosporidium log removal credit greater than 4 log (per EPA LT2ESWTR); Giardia greater than 4 log. Chemical pre-coagulation before membrane (inline coagulation) improves NOM (natural organic matter) removal and membrane fouling control. Membrane integrity testing: daily pressure decay test (PDT, also known as PDRT) at 10 to 30 kPa applied transmembrane pressure; integrity threshold corresponds to 3 to 4 log Cryptosporidium removal credit per US EPA IT/CR guidance. UK DWI requires continuous turbidity monitoring at membrane plant effluent (less than 0.1 NTU); breach triggers automatic process alarm and investigation. Alternative turbidity removal for small systems: slow sand filters (SSF, BS EN 1988:2019, loading rate 0.1 to 0.3 m/hour, 0.15 to 0.3 mm sand) provide biological turbidity removal through the Schmutzdecke biological layer; achieve less than 1 NTU from source water less than 10 NTU; requires pre-sedimentation for higher source turbidity.
Frequently Asked Questions
What turbidity level is safe for drinking water?
WHO Guideline: turbidity less than 1 NTU at the point of chlorination for effective disinfection; less than 0.1 NTU for optimal UV disinfection efficacy (turbidity above 1 NTU can reduce UV transmittance sufficiently to prevent meeting CT requirements at standard UV doses). UK Water Supply (Water Quality) Regulations 2016: mandatory standard 4 NTU at consumer tap; UK water treatment best practice targets less than 0.5 NTU at treatment works outlet and less than 0.1 NTU for membrane plants. US EPA: Greater than 1 NTU triggers investigation at conventional plants (SWTR); Greater than 0.3 NTU requires reporting and corrective action for filtration plants under ESWTR. High turbidity correlation with risk: NTU greater than 10 in finished water indicates probable filter breakthrough or process failure requiring immediate investigation. Aesthetic: consumers perceive turbidity greater than 4 NTU as visibly cloudy; greater than 1 NTU is detectable by sensitive observers. Turbidity monitoring: online continuous monitoring (nephelometer, ISO 7027, 90-degree scatter) at filter effluent and service reservoir outlet; DWI requires alarm-triggered investigation at 0.4 NTU for UK surface water treatment works.
How does coagulation remove turbidity?
Coagulation removes turbidity by destabilising colloidal particles so they can aggregate into settleable flocs. Colloids (0.001 to 1 micron) have negative surface charge (Zeta potential -20 to -60 mV for natural clay and humic colloids) which causes electrostatic repulsion preventing aggregation. Coagulants (aluminium sulphate, ferric sulphate, PACl) hydrolyse to form positively charged metal hydroxo species and metal hydroxide precipitate: (1) Charge neutralisation: coagulant cations adsorb on negative particle surfaces, reducing Zeta potential toward zero (-5 to +5 mV) removing electrostatic barrier to aggregation; optimum dose gives Zeta potential near-zero; (2) Sweep flocculation: at higher doses, voluminous metal hydroxide floc forms and physically enmeshes particles regardless of their surface charge - effective for high-turbidity, high-colour water. Jar test: standard coagulation optimisation tool; 1-litre glass jars; 6 doses of coagulant; 60 seconds at 200 RPM rapid mix; 20 minutes at 30 RPM slow mix; 15 minutes settling; measure supernatant turbidity and UV254 (colour proxy); select dose giving minimum turbidity and UV254 residual. PACl (polyaluminium chloride): pre-polymerised; less pH-sensitive than alum; effective at lower temperatures (less than 5 degrees C where alum hydrolysis slows); preferred for cold highland reservoirs.
Can ultrafiltration membranes remove turbidity below 0.1 NTU?
Yes. Ultrafiltration (UF) membranes with MWCO 10,000 to 100,000 Dalton and nominal pore size 0.01 to 0.1 micron provide absolute removal of all particles larger than the rated pore size. Turbidity removal performance: UF effluent turbidity consistently less than 0.1 NTU regardless of feed water turbidity (1 NTU or 100 NTU feed: same effluent less than 0.1 NTU); typically less than 0.05 NTU. Particle removal credit: UF membranes with greater than 4 log Cryptosporidium removal credit (US EPA LT2ESWTR); greater than 4 log Giardia; greater than 3 log virus (larger pore UF); greater than 6 log for high-integrity, low-MWCO UF systems. UK DWI: UF membranes with appropriate integrity testing credited with 3 to 4 log Cryptosporidium reduction under DWI Technical Guidance (UKWIR report on membrane risk assessment). Key requirement: continuous integrity testing (pressure decay test, PDT, typically daily or after each backwash cycle) confirming no membrane fibre integrity failure; automated breach detection with process alarm. Limitations: UF removes particles but not dissolved contaminants (DOC, NOM, ions); post-UF disinfection (chlorination, UV) still required. UF effluent NOM (UV254): same as feed unless pre-coagulation is applied; coagulation before UF (inline coagulation) removes NOM adsorbed on floc and reduces DBP precursors.
What is the difference between slow sand filtration and rapid gravity filtration?
Slow sand filtration (SSF) and rapid gravity filtration (RGF) are both granular media filtration processes but operate on entirely different principles: SSF: filtration rate 0.1 to 0.3 m/h; sand size 0.15 to 0.30 mm; operates by biological filtration through the Schmutzdecke (German: 'dirty skin') - a 5 to 30 mm biologically active layer of algae, bacteria, protozoa, and organic matter on the sand surface; removes turbidity (less than 1 NTU from less than 10 NTU feed), bacteria (3 to 4 log), Giardia and Cryptosporidium (2 to 4 log), and some organics; no chemical addition; low energy; requires 10 to 30 times the land area of RGF for equivalent throughput; resanding every 1 to 5 years when head loss increases. RGF: filtration rate 5 to 15 m/h (50 times faster than SSF); dual-media (anthracite 1.0 to 1.4 mm / sand 0.5 to 0.85 mm); operates by physical-mechanical filtration (straining, sedimentation within pores, adsorption on media); requires pre-coagulation for turbidity removal; backwash every 24 to 72 hours by air scouring + upflow water (10 to 15 percent of throughput wasted); achieves less than 0.2 NTU. BAC (biological activated carbon) = RGF post ozonation with biological activity - combines RGF physical removal with biodegradation of organic micropollutants.
A water company in Wales operating a 40 MLD upland reservoir treatment works was experiencing seasonal turbidity events where raw water turbidity spiked above 200 NTU during storm events, overwhelming the existing coagulation-flocculation system and causing treated water turbidity to breach the DWI limit of 1 NTU. Two episodes in two years had triggered DWI Water Quality Notices.
The company upgraded the coagulation stage by installing variable-speed polymer dosing and an online zeta potential meter for coagulation control, allowing real-time optimisation of ferric sulphate dose (increased from 15 to 45 mg/L during storm peaks). A ballasted flocculation stage (Actiflo, microsand-weighted floc, 10-minute HRT) was retrofitted upstream of the existing RGF media filters. The RGF backwash cycle was automated via turbidimeter feedback rather than a fixed time interval.
Treated water turbidity during the next two high-turbidity events (120 NTU and 310 NTU raw water) was maintained at 0.08 NTU and 0.12 NTU respectively, well within the 1 NTU DWI limit. DWI confirmed closure of the outstanding Water Quality Notice. Chemical dose optimisation reduced ferric sulphate consumption by 18% in normal operation through tighter zeta potential control. Two RGF backwash water recovery was also improved, reducing washwater waste by 12%.
Questions to Ask Shortlisted Providers
- 1
What is the maximum and typical turbidity of the raw water source and what is the minimum and maximum flow rate to be treated?
Treatment train selection (conventional RGF, ballasted flocculation, UF/MF membrane) depends on raw water turbidity variability; high peak-to-average turbidity ratios favour ballasted flocculation or membrane filtration over conventional settlement.
- 2
What are the DWI or EA compliance targets for treated water turbidity and what is the performance history against these targets?
DWI requires less than 1 NTU at the treatment works outlet under WS(WQ)R 2016; repeated failures trigger Water Quality Notices and escalating DWI scrutiny; understanding the frequency and duration of failures shapes the design standard.
- 3
What coagulant type and dose has been used historically and is real-time coagulation control (streaming current, zeta potential, UV254) implemented?
Fixed coagulant doses cause over-treatment at low turbidity (sludge generation cost) and under-treatment at high turbidity (compliance risk); real-time control reduces both risks and typically saves 10 to 25% on coagulant chemical cost.
- 4
Is Cryptosporidium risk assessment required and does the treatment works need to comply with the Cryptosporidium in Water Supplies Regulations 1999?
Surface water abstractions are subject to DWI Cryptosporidium Risk Assessment requirements; turbidity removal is a critical barrier to Cryptosporidium; media filters must achieve minimum log credit established in the risk assessment; additional barriers (UV or membrane) may be required.
- 5
What is the backwash water recovery or disposal route and what are the sludge management obligations?
RGF backwash water contains coagulated solids and returned chemicals; recovery to inlet reduces water loss (typically 3 to 8% of throughput); backwash sludge disposal must comply with EA permit conditions or trade effluent consent.
What Drives Cost in This Category
Ferric sulphate or PAC coagulants cost GBP 80 to 250 per tonne; for a 40 MLD works at 20 mg/L average dose, chemical cost is GBP 220 to 700 per day; automated streaming current or UV254 control reduces average dose by 15 to 25% without compliance risk.
Dual-media (anthracite + sand) replacement for an RGF filter costs GBP 25,000 to 80,000 per filter cell including disposal and reinstallation; media life 15 to 25 years with proper backwash management; undersized or incorrectly operated backwash causes premature media attrition.
WTW sludge (coagulated solids, decanted backwash) must be dewatered to 15 to 30% DS; filter press or centrifuge dewatering cost GBP 60 to 120 per tonne DS including disposal; recycle of backwash water reduces sludge volume but increases inlet organic loading.
WS(WQ)R 2016 requires regular sampling at defined points; compliance monitoring costs GBP 30,000 to 150,000 per year for a large works including UKAS-accredited laboratory analysis, turbidimeter calibration, and DWI annual return preparation.
Key Regulations & Standards
Prescribes the turbidity parametric value of 1 NTU for drinking water at treatment works outlets and at consumers' taps; repeated exceedances trigger DWI enforcement under Regulation 28 (investigation) and Regulation 31 (undertaking notices); compliance is assessed on all samples taken.
Require water companies to undertake Cryptosporidium Risk Assessments for all surface water and groundwater-under-direct-influence-of-surface-water sources; treatment works must achieve the log credit required by the risk assessment; continuous turbidity monitoring with automated process shutdown is required.
DWI guidance specifies coagulation performance standards, jar testing requirements, real-time monitoring expectations, and the documentation required for Treatment Works Risk Assessment under WS(WQ)R 2016 Schedule 8.
EA abstraction licences include conditions on minimum abstraction rates and source protection requirements; WTW sludge (backwash water, coagulation sludge) discharge to watercourse or land requires an Environmental Permit or Regulatory Position Statement compliance.