Industrial Effluent from Construction and Demolition: pH, Suspended Solids and Oil Interceptor Design

What counts as industrial effluent on a construction site?

Unlike a factory with a fixed process, a construction and demolition site’s effluent changes daily as activities progress — from concrete pouring to demolition dust to vehicle washdown. The common streams are these:

The Environment Agency’s default position is that all water leaving a site boundary, other than clean roof runoff, is either trade effluent if it goes to sewer or a controlled discharge if it goes to a watercourse or ground — and needs a permit or consent either way. Groundwater inflows are covered separately in our guide to construction dewatering.

The regulatory framework

A trade effluent consent is required for concrete washout water even after pH adjustment, for vehicle wash water, for treated TBM slurry, and for any water above 1,000 mg/L suspended solids, above 15 mg/L oil, or outside pH 6–10.

pH control: cementitious washout and lime

Concrete washout, cement grout and lime-stabilised soil produce effluent at pH 11–13. The chemistry is dominated by calcium hydroxide: a saturated solution gives roughly 0.011 M hydroxide, which is pH 12.0. Sodium and potassium hydroxides used as accelerators can push it past 13. At those pH values, chromium VI leaches out of the cement — a carcinogen regulated at 50 µg/L in drinking water.

Sulphuric acid is the usual choice on cost and availability, but calcium sulphate precipitation is a real risk: sulphate from neutralisation meeting residual calcium can exceed the solubility product and scale pipes and tanks. Watch conductivity — a sudden rise signals precipitation. The control strategy is covered in pH neutralisation systems, and the dosing hardware in chemical dosing system design.

For a continuous flow such as a batching plant, CO₂ is safer and adds no salt: it converts hydroxide to bicarbonate and is self-limiting near pH 8.3, so it cannot overshoot into acid. It costs several times more than sulphuric acid per batch, but removes the corrosion, scaling and handling risks.

Suspended solids: silt, grout and demolition debris

The controlling parameter for a settlement pond is the overflow rate, v_o = Q / A. Sand needs below 5 m/h; silt below 0.5 m/h; clay and colloids would need below 0.05 m/h, which is impractical — at that point coagulation is not optional.

Cement particles carry a negative surface charge above pH 10, so ferric chloride works on three fronts at once: charge neutralisation, sweep flocculation as ferric hydroxide precipitates and enmeshes the fines, and pH reduction, because the coagulant is itself acidic. A jar test at 20–120 mg/L as Fe typically finds that 60–80 mg/L takes solids from 15,000 to below 500 mg/L and pH from 12.5 to 8.5–9.0 in a single stage. The method is set out in coagulation and flocculation.

The solids removed have to go somewhere: a clarifier and filter train produces 2–5% of throughput as sludge. See sludge dewatering equipment for the options.

Oil and hydrocarbon interception

The pollution prevention guidance requires an oil separator wherever vehicles are refuelled, have oil changed, or are washed; wherever large numbers of vehicles park or manoeuvre daily; and wherever there is a high risk of oil contamination such as near fuel storage. Most construction sites meet several of those criteria at once. Even washing a single concrete mixer daily produces water carrying diesel, hydraulic fluid and engine oil above sewer consent limits.

Class I units treat to 5 mg/L and are required for discharge to sensitive watercourses; Class II units at 100 mg/L are acceptable to foul sewer with consent. Sizing and selection are covered in oil and grease separators.

Separators fail by sediment accumulation and oil saturation, so inspect monthly, desilt when sediment reaches 30% of volume — typically every three to six months on a C&D site — and remove oil when the layer exceeds 10% of volume. Separator sludge above 0.1% oil is hazardous waste (16 07 08*) and needs a licensed carrier.

Putting the train together

A concrete batching plant illustrates the integration problem: truck washout at 10 m³/day, pH 12.5 and SS 20,000 mg/L; mixer washdown at 5 m³/day, pH 11 and SS 10,000 mg/L; and yard runoff at up to 50 m³/day during storms, pH 8 and SS 500 mg/L. Three streams, wildly different in strength and utterly different in timing.

The clarifier and filter produce roughly 0.3–0.8 m³/day of sludge at 3% dry solids. Because it is pH-adjusted cement sludge carrying oil, it is hazardous waste: dewater it on site and dispose through a licensed carrier.