When an 18.9 m³ brewery DAF could not handle high MLSS
This full-scale record is valuable because it documents an underperforming DAF rather than presenting only a successful commissioning. The brewery used activated sludge followed by DAF for secondary solids separation. The unit was nominally large enough on hydraulic flow, but the actual mixed-liquor solids concentration and flotation behaviour reduced usable capacity to a fraction of the rating.
A 5,000 US gal (18.9 m³) DAF rated at 150 gpm (818 m³/day) and 5,000 mg/L TSS could process only 20.2 gpm (about 110 m³/day) at the observed 7,250 mg/L TSS. Average effluent was 164 mg/L TSS and 354.8 mg/L COD, but severe upsets made the system unreliable.
Wastewater and biological stage
Raw brewery wastewater passed through a 40,000 US gal (151 m³) equalisation tank and then an 80,000 US gal (303 m³) aeration tank. The DAF was not treating screened raw wastewater; it was separating activated sludge at an average MLSS of approximately 6,709 mg/L. The published one-year raw feed averages were exceptionally strong.
| Parameter | Published brewery feed | Published DAF effluent average |
|---|---|---|
| COD | 11,716 mg/L | 354.8 mg/L |
| Soluble COD | 5,113 mg/L | Not separately reported at DAF outlet |
| TSS | 7,246 mg/L | 164 mg/L |
| pH | 5.4 | 7.2 |
| Temperature | 86.5 °F / 30.3 °C | 99.3 °F / 37.4 °C |
DAF configuration and actual duty
| Item | Design or reported value |
|---|---|
| DAF vessel | 5,000 US gal (18.9 m³), 12 ft (3.66 m) high |
| Nameplate basis | 150 gpm (818 m³/day) at 5,000 mg/L TSS |
| Observed full-capacity operation | 20.2 gpm (approximately 110 m³/day) at 7,250 mg/L TSS |
| Whitewater arrangement | DAF effluent saturated with air in an air receiver and sparged into the bottom of the unit |
| Surface solids removal | Mechanical skimmer; baffle separated floated sludge from clarified underflow |
| Chemistry | Average 2.3 US gal/day polymer and 9.9 US gal/day coagulant were reported |
| Sludge dewatering | Screw press produced 1–5% solids, averaging 2.8% |
Measured performance—and the instability hidden by averages
Across the complete biological-plus-DAF train, the paper calculated 97.0% COD removal and 97.7% TSS removal. Those annual averages look strong. Operationally, however, the DAF suffered poor floc flotation, foaming, overflows and batches that had to be removed by tanker.
The maximum reported DAF-effluent TSS was 5,210 mg/L and the maximum COD was 6,000 mg/L. Chemical spending was approximately US$10,000 per month, while unprocessable material was hauled away at about 10,000 US gal/day during problem periods. The plant eventually replaced the DAF clarification stage with an MBR.
Engineering lesson for brewery projects
This case does not show that DAF is unsuitable for breweries. It shows that hydraulic capacity cannot be separated from solids mass loading, sludge properties and upstream biology. A unit rated for 150 gpm at 5,000 mg/L was not a 150 gpm unit when fed 7,250 mg/L of difficult activated sludge.
For secondary clarification, a proposal should state at least the design MLSS range, kg TSS/h, recycle ratio and pressure, air-to-solids ratio, flocculation conditions, surface loading, sludge-removal rate and turndown. A raw-water flow alone is not an adequate DAF specification.
The engineering figures were checked against a technical paper documenting one year of full-scale operation. Brewery, supplier and location details are intentionally withheld. Imperial-to-metric conversions are rounded; DAF outlet values are reported averages, and whole-train removal percentages include upstream biological treatment.