Status: Delivered · Region: North America · Ref. D-040
What We Delivered
We delivered a dissolved air flotation (DAF) unit with automatic coagulant/flocculant dosing, installed upstream of an existing on-site lagoon at a truck stop in North America. The client is a truck-stop operator with several food-service outlets and fuel sales on one site; its identity, the site and the contract terms are withheld under our client-confidentiality policy.
The site discharged roughly 10,000 US gallons per day (~40 m³/day class) into a lagoon system that was failing its discharge tests and was being tankered off site daily. The design keeps the existing lagoon in service downstream of the DAF instead of rebuilding it. No influent analysis was available when the plant was specified, so the chemical programme was designed to be tuned on site after start-up.
| Parameter | Value |
|---|---|
| Site type | Truck stop: food service + fuel sales |
| Flow | ~10,000 US gal/day (~40 m³/day class) |
| Existing system | Lagoon, hydraulically and organically overloaded |
| Condition before the project | Failing discharge tests; tankered off site daily |
| Climate | Continental, sub-zero winters |
| Equipment delivered | DAF unit with automatic chemical dosing |
Source: client-supplied site description. No influent analysis was available when the plant was specified — a common situation in emergencies, and one that shaped the engineering approach below.
Delivery Record
The project ran through nine delivery stages. The table lists what each stage covered on this project and the record it produced.
| Stage | What it covered on this project | Record produced |
|---|---|---|
| 1. Design basis confirmation | Site flow (~10,000 US gal/day, ~40 m³/day class), meal-period peaking (factor about 2.5–3, design flow 4–5 m³/h), sub-zero winter conditions and retention of the existing lagoon. The absence of influent analysis was recorded, with the chemical programme to be tuned on site. | Design basis sheet |
| 2. Process & mechanical design | DAF flotation area at a surface loading of 3–5 m³/m²·h for the 4–5 m³/h design flow, 20–30% pressurised recycle, adjustable coagulant/flocculant dosing, and a layout inside heated space with freeze protection on all external lines. | Process calculation sheet, P&ID, GA and layout drawings |
| 3. Manufacturing & quality control | Fabrication of the DAF unit, its saturation system and the chemical dosing system. | Material and quality-control inspection records |
| 4. Factory acceptance test (FAT) | Pre-dispatch function check of the DAF, the recycle saturation system and the dosing pumps. | FAT record |
| 5. Packing & dispatch | DAF unit and dosing system packed and dispatched for shipment to site. | Packing list and dispatch documents |
| 6. Installation (per contract scope) | Installation inside heated space with freeze protection on all external lines, as fixed at the layout stage. | Installation record |
| 7. Commissioning | Start-up of the DAF and dosing system; commissioning scope included on-site tuning of the coagulant/flocculant programme against real influent samples. | Commissioning log |
| 8. Performance verification | DAF operation at the design flow checked against the design basis. | Performance test record against the design values |
| 9. Training & handover / after-sales | Operator training on the dosing programme, float removal and sludge haul-off, and winter freeze-protection checks. After-sales support per contract scope. | O&M manual, training record, handover record |
Delivery records for this project are on file and can be reviewed by qualified buyers under a confidentiality agreement.
Designed For
- Sub-zero winters — dosing lines, saturation system and instrument tubing protected against freezing.
- FOG-heavy food-service flows — discharge concentrated around meal service, giving a 4–5 m³/h design flow.
- An existing lagoon kept in service — downstream of the DAF, instead of being rebuilt.
- No influent analysis at specification — adjustable coagulant and flocculant dosing.
- An emergency timeline — a physical-chemical process that works from start-up while tankering costs were accruing daily.
Why the Lagoon Failed — and Why That Matters Before Buying Anything
The instinct in this situation is to replace the lagoon. That is usually the wrong first move, and it is the expensive one.
A lagoon serving several food-service outlets receives a load profile it was probably never designed for: fats, oils and grease (FOG) from commercial kitchens. FOG does not behave like dissolved organic load. It floats, it congeals — particularly in cold climates — and it forms a surface mat. That mat blocks the atmospheric re-aeration a facultative lagoon depends on. Oxygen transfer collapses, the aerobic layer thins, and the lagoon stops treating long before it runs out of hydraulic volume.
The visible symptom is "the lagoons are full". The actual failure is usually oxygen transfer, not volume. Which means the correct question is not "how big a replacement plant do we need" but "what do we remove upstream so the existing asset works again?"
The Engineering Answer: Intercept FOG and Solids Before the Lagoon
Dissolved air flotation is well matched to this duty. It removes FOG and suspended solids by attaching microbubbles to them and floating them off — no biology involved, so it starts working within hours of commissioning rather than the weeks an activated-sludge process needs to seed.
For an emergency, that difference matters more than any efficiency figure.
| Design item | Basis | Value |
|---|---|---|
| Average flow | ~10,000 US gal/day (~40 m³/day class) ÷ 24 h | ≈ 1.6–1.7 m³/h |
| Design flow (peak meal periods) | Peaking factor ~2.5–3 | 4–5 m³/h |
| DAF surface loading | Standard range 3–5 m³/m²·h | — |
| Recycle ratio (pressurised saturation) | Typical 20–30% of feed | — |
Sizing to peak rather than average is what stops a restaurant DAF from being undersized. Kitchen discharge concentrates around meal service; a unit sized on the 1.6–1.7 m³/h daily average would be hydraulically overrun exactly when the FOG load arrives.
Source: HydropureWater design calculation, starting from the site flow (~10,000 US gal/day, ~40 m³/day class). Surface loading and recycle ranges are standard DAF design values.
Two Constraints Specific to This Site
1. No influent data — so the chemical programme was made adjustable
Restaurant and truck-stop wastewater varies widely with menu, dishwashing practice and grease-trap maintenance. Rather than design against an assumed strength, the plant was specified with adjustable coagulant and flocculant dosing so the chemical programme could be tuned against real influent once the unit was running.
2. Sub-zero winters
Chemical dosing lines, saturation systems and instrument tubing are the first casualties of a hard freeze, and a DAF that freezes solid in January is worse than no DAF at all. Installation was specified inside heated space with freeze protection on all external lines — a requirement that materially affects layout, so it was fixed at the layout stage rather than after delivery.
Process Arrangement
Grease interceptor → equalisation → DAF with coagulation/flocculation → existing lagoon (retained) → discharge. Floated sludge is thickened and hauled off site.
The lagoon stays in service. The design intent is that, relieved of the FOG and suspended-solids load, its surface can re-aerate and its remaining biological capacity is applied to the dissolved organic load it was built for. This makes the project an upgrade of an existing asset rather than a capital replacement.
Equipment Delivered
- Dissolved air flotation (DAF) unit — FOG and suspended-solids removal.
- Automatic chemical dosing system — coagulant/flocculant dosing, adjustable against measured influent.
For DAF sizing methodology in more depth, see our DAF sizing guide.
For Similar Projects
- Establish the failure mode before budgeting for a new plant: on sites with commercial kitchens, a failing lagoon is usually an oxygen-transfer problem, not a volume problem.
- Size the DAF on meal-period peak flow, not the daily average, and confirm the peaking factor against the site's actual service pattern.
- Where winters are sub-zero, settle heated installation space and freeze protection on external lines at the layout stage, before the unit ships.
- If no influent analysis is available, specify adjustable coagulant/flocculant dosing and tune the programme against real samples during commissioning.
- Do not accept a fixed removal percentage quoted for a site with no influent analysis; nobody can stand behind that number.
Frequently Asked Questions
Our lagoon is full and failing tests — do we have to replace it?
Often not. On sites with commercial kitchens, lagoon failure is usually caused by a FOG surface mat blocking re-aeration rather than by a genuine shortage of volume. Removing FOG and suspended solids upstream can let the existing lagoon keep working, and is a smaller capital step than replacement. Establish the failure mode before you budget for a new plant.
How fast can a DAF be commissioned in an emergency?
A DAF is a physical-chemical process, so it produces effect from the first hours of operation — unlike a biological plant, which needs weeks to establish a viable population. That is precisely why it suits emergency interventions where tankering costs are accruing daily.
What size DAF does a 10,000 gallon/day restaurant site need?
Size on peak flow, not the daily average. A site discharging about 10,000 US gal/day (~40 m³/day class) averages roughly 1.6–1.7 m³/h, but kitchen discharge concentrates around meal service, giving a realistic design flow of 4–5 m³/h. At a standard surface loading of 3–5 m³/m²·h, that sets the flotation area.
Will a DAF work in a climate with sub-zero winters?
Yes, provided freeze protection is designed in rather than added later. Dosing lines, the saturation system and instrument tubing are the vulnerable components. Heated installation space and traced external lines should be settled at the layout stage — retrofitting freeze protection after delivery is far more disruptive.
Do we need influent testing before ordering?
Ideally yes, but emergencies rarely allow it. The workable alternative is to specify adjustable chemical dosing and tune the programme against real samples during commissioning. What you should not accept is a guaranteed removal percentage quoted against water nobody has analysed.
