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Fats, oils, and grease represent one of the most challenging pollutant classes in food manufacturing effluent, requiring targeted removal and presenting significant recovery value.
Fats, oils, and grease (FOG) constitute a major pollutant load in food processing wastewater, originating from edible oil refining, animal rendering, dairy cream, frying operations, and meat trimming. These hydrophobic compounds exist in free-floating, emulsified, and bound forms, each presenting distinct removal challenges. Free FOG can be separated by gravity, but emulsified fats stabilised by proteins and surfactants require chemical destabilisation followed by dissolved air flotation (DAF) for effective recovery.
Regulatory limits for FOG discharge are stringent: typically <15 mg/l for direct discharge to watercourses under Environment Agency permits, and <100 mg/l for sewer discharge under trade effluent consents. Exceeding these limits risks prosecution, sewer surcharge penalties, and loss of discharge consent. Beyond compliance, FOG causes severe operational problems including pipe blockages, pump fouling, inhibition of advanced biological treatment processes, and formation of surface scum in settling tanks that degrades effluent quality.
The recovery value of separated fat is substantial and often overlooked. Recovered animal fat and vegetable oil can be sold to rendering companies or biodiesel producers at -400 per tonne, transforming a waste disposal liability into a value stream. A well-designed DAF system captures 85-95% of influent FOG as a concentrated float sludge with 15-30% dry solids oil content, suitable for direct collection by rendering contractors.
Dissolved air flotation is the industry standard for FOG removal from food wastewater, achieving 90-99% efficiency when properly designed with appropriate chemical conditioning. DAF works by saturating a portion of the treated water with air under pressure, then releasing it at atmospheric pressure to generate micro-bubbles (20-50 µm) that attach to hydrophobic fat droplets, floating them to the surface for mechanical skimming. Reynolds & Bauhm is involved in the design and manufacture of DAF units specifically optimised for food industry FOG applications, including heated configurations for high-melting-point animal fats.
The key design parameters for food FOG DAF include hydraulic loading rate (typically 5-12 m/h), recycle ratio (15-30% depending on FOG concentration), and saturator pressure (4-6 bar). Our food industry DAF units feature stainless steel wetted parts for corrosion resistance, insulated tanks for temperature maintenance, and variable-speed scrapers to handle fluctuating float thickness. Automatic sludge level control and oil-weir skimming ensure consistent performance across shift patterns and cleaning cycles.
Regulatory context: UK water companies impose FOG limits under Section 111 of the Water Industry Act 1991. Discharges exceeding 100 mg/l FOG to sewer risk refusal of consent, while direct discharges to controlled waters require <15 mg/l under Environmental Permitting Regulations. Prosecution for non-compliance can reach unlimited fines. A properly designed DAF system provides a robust defence against enforcement action while generating recoverable product.
FOG concentration, physical form, and recovery value vary dramatically across food sectors. Correct characterisation determines DAF configuration and economic viability.
| Food Sector | FOG Range (mg/l) | FOG Type | Recovery Value | Best DAF Config |
|---|---|---|---|---|
| Edible Oil Processing | 500 – 5,000 | Vegetable oil | High | High-rate DAF |
| Meat / Rendering | 300 – 2,500 | Animal fat | High | Heated DAF |
| Dairy / Cheese | 150 – 800 | Butterfat | Medium | Standard DAF |
| Snack Foods | 100 – 600 | Frying oil | Medium | Standard DAF |
| Bakery | 50 – 400 | Shortening | Low | Compact DAF |
| Ready Meals | 100 – 600 | Mixed | Low | Standard DAF |
Animal fats and vegetable oils command the highest rendering values. Bakery and ready-meal FOG is typically too degraded or mixed for economic recovery but still requires removal for compliance.
Emulsion types matter. Free FOG separates in 10-30 minutes by gravity and is easily captured. Emulsified FOG, stabilised by proteins, phospholipids, or surfactants from cleaning chemicals, will not separate without chemical coagulation. Bound FOG adheres to suspended solids and is removed with the solid phase. Our jar test protocol distinguishes these forms on-site, ensuring the correct chemical programme and DAF loading rate are selected for each facility.
A five-stage Treatment Process optimised for fat separation, from equalisation and heating through to skimming and sludge removal.
Balance flow and load variations. Heat to 40-60°C for animal fats to reduce viscosity and prevent solidification in pipes and the DAF cell.
Adjust pH to 6.5-7.5 to optimise zeta potential reduction and coagulant performance. Emulsified fats are most effectively destabilised in this range.
Dose FeCl3 coagulant to collapse emulsion charge, followed by cationic PAM flocculation to aggregate fat droplets into larger flocs.
Micro-bubbles (20-50 µm) generated by pressure release attach to hydrophobic fat flocs. Bubble-particle attachment follows Young-Laplace principles.
Mechanical flight scrapers remove the concentrated fat float. Bottom scrapers remove settled solids. Float sludge contains 15-30% DS oil.
Bubble-particle attachment theory: DAF removal efficiency depends on the probability of collision between micro-bubbles and fat droplets, their subsequent attachment governed by interfacial tension, and the rising velocity of the aggregate. Reynolds & Bauhm DAF saturators produce a consistent bubble size distribution centred on 30-40 µm, maximising attachment probability for typical food industry fat droplets (50-200 µm). The three-phase contact angle between air, oil, and water favours strong attachment, making FOG one of the most DAF-amenable contaminants.
Why DAF over gravity separation? Gravity API or plate separators achieve 60-70% FOG removal at best and require 4-10 times the footprint of an equivalent DAF. They also fail completely on emulsified FOG. DAF achieves 90-99% removal in a compact footprint by actively floating particles rather than relying on passive density difference. For food wastewater where FOG is often emulsified and space is at a premium, DAF is the only technology that reliably delivers compliance.
Precise chemical selection and dosing transforms DAF performance from marginal to exceptional. Reynolds & Bauhm systems include automated dosing control.
FeCl3 destabilises fat emulsions by neutralising negative surface charge on oil droplets and proteins. Typical dose: 100-400 mg/l depending on emulsion strength. Enables phase separation that gravity alone cannot achieve.
High-molecular-weight cationic PAM bridges destabilised fat droplets into large, buoyant flocs. Dose typically 2-8 mg/l. Molecular weight 8-12 million Da optimal for DAF float formation.
Food fat emulsions typically carry a negative zeta potential of -30 to -50 mV at neutral pH. Coagulant demand drops significantly as pH approaches the isoelectric point of adsorbed proteins (pH 6.0-6.8).
Heating wastewater to 40-60°C reduces water viscosity by 40-60%, accelerates bubble rise velocity, and prevents animal fat solidification. Our heated DAF units include insulated tanks and steam injection.
We conduct on-site jar testing to determine optimal coagulant type, dose, pH, and flocculant selection. Results directly translate to automated dosing setpoints, ensuring efficient chemical consumption.
DAF float sludge from food FOG contains 15-30% dry solids, of which 60-80% is recoverable oil. This high oil fraction makes the sludge valuable to renderers and distinguishes it from low-grade biological sludge.
Chemical storage and handling: Ferric chloride is supplied in IBCs or bulk tanks with secondary containment. Cationic PAM is prepared as a 0.1-0.3% solution in automated polymer stations. Both chemicals require standard PPE and bunding. Reynolds & Bauhm is involved in supplying complete dosing skids with day tanks, metering pumps, and leak detection as standard. Dosing is PLC-controlled with flow-proportional feed and pH trim to minimise over-dosing and chemical requirements.
Quantify the financial case for DAF-based FOG recovery: rendering output, disposal benefits, and sewer compliance benefits.
FOG Loading
kg/day = Flow (m³/day) × FOG (mg/l) / 1000
Recovery Rate
85-95% of influent FOG
Sludge Yield
3-5% of flow volume
Scenario: 500 m³/day flow at 800 mg/l FOG
Disposal reduction: Without DAF recovery, FOG-laden sludge management -250/tonne to dispose of. DAF concentrate sent to rendering is typically collected free or at a rebate. Sewer surcharge for FOG >100 mg/l can exceed for a 500 m³/day facility.
For a mid-sized edible oil or meat processing facility, combined rendering output (-80,000/year), disposal benefits (-15,000/year), and sewer compliance benefits (-50,000/year) frequently deliver a project project benefits of 2-4 years on Capital expenditure investment. Maintenance costs are low: scraper chain replacement every 5-7 years and saturator pump servicing annually.
Scenario: 100 m³/day bakery at 300 mg/l FOG. Loading = 30 kg/day. At 85% recovery over 250 days = 6,375 kg/year. At (degraded shortening) =. However, sewer compliance benefits (-8,000/year) and blocked drain prevention provide the dominant economic justification for compact DAF installation in smaller facilities.
Realistic Capital expenditure and Operating expenditure figures for typical food industry FOG recovery DAF installations, based on recent proposals.
Includes sludge thickening and dedicated oil recovery tank with heating.
Timeline: 14-18 weeks design to commissioning. Process guarantee: FOG <20 mg/l.
Includes screening, heated DAF, screw press, and fat storage tank with agitation.
Timeline: 12-16 weeks. Process guarantee: FOG <25 mg/l with 90%+ fat recovery.
Includes compact DAF, chemical dosing skid, and automated skimming system.
Timeline: 10-14 weeks. Ideal for retrofit into existing WWTP with minimal civil works.
Figures include design, manufacture, installation, and commissioning. Chemical requirements based on bulk ferric chloride and powder PAM supply. Recovery output assumes rendering contract at -0.35/kg. Operating expenditure excludes labour and power (< for a 200-400 m³/day unit).
Our DAF systems deliver measurable operational, financial, and compliance advantages for food processors.
Reynolds & Bauhm DAF systems for FOG recovery are installed across the UK and Europe in facilities ranging from 50 m³/day artisan dairies to 2,000 m³/day edible oil refineries. Every system is designed, manufactured, and commissioned with Reynolds & Bauhm involvement with process guarantees on FOG removal performance.
Consistently meet the strictest direct discharge consent limits and trade effluent FOG requirements with 90-99% removal efficiency.
Transform waste FOG into a value stream. Recovered fat sales to rendering or biodiesel can offset 20-40% of treatment Operating expenditure.
Avoid Mogden formula surcharges and trade effluent penalties by reducing FOG to consent-compliant levels before sewer discharge.
Remove FOG at source before it solidifies in drains, pipes, and sumps. Eliminate costly emergency drain jetting and production downtime.
Prevent FOG inhibition of aerobic bacteria and biofilm carriers. Maintain stable advanced biological treatment performance and effluent quality.
DAF units provide high-rate separation in a small footprint. Ideal for retrofit into existing plants where space is limited.
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View PageRecovered FOG is a value stream; the chemistry of emulsion breaking determines yield.
Free oil (gravity-separable, >150 μm); dispersed oil (20–150 μm, settles slowly); emulsified oil (<20 μm, stable). Each form needs a different removal mechanism.
Surfactants (CIP detergents), proteins (casein, gluten), phospholipids (lecithin), salts. Each protects droplets electrostatically or sterically. Identify before specifying coagulant.
Mechanisms: charge neutralisation (cationic polymer 5–25 mg/L); pH shift (drop below 5.0); thermal (>70°C for 20 min); polyaluminium / ferric coagulation 100–300 mg/L.
Hydraulic loading 4–8 m³/m²·h. A/S ratio 0.025–0.04. Saturator 5–6 bar. Recycle ratio 30–45%. Achievable: 95–99% FOG removal.
Float sludge 3–15% solids. With dewatering (centrifuge or filter press): 25–45% solids. Cake sold to renderers at –120/t depending on FOG content and contamination.
pH shock (drop to 5.0) for emulsion breaking requires NaOH re-correction before biological stage. Pair with pH correction dosing, see comparative practice in food & beverage and oil-gas reference designs.
Contact Our Engineers for a site assessment, jar testing, and a tailored DAF proposal for your food processing facility.
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