The clarification is set out in a regulatory position statement (RPS) entitled “Managing concrete wash water on construction sites: good practice and temporary discharges to ground or to surface waters”*.
While the RPS provides clear guidance on the environmental permitting and treatment needs for the disposal of wash water generated from rinsing truck mixer chutes, Dr Coulton is concerned that time-pressured site managers may be tempted to ignore the advice and risk fines or prosecution. “Construction managers need to understand how this affects their sites and plan ahead to ensure the treatment and disposal of concrete wash water is managed professionally,” he comments.
Different environmental permitting and treatment needs are based on the number of truck mixer movements and, hence, the estimated volume of high pH wash water generated. Under the RPS, sites are classified as small, medium, large or very large depending on the number of concrete deliveries received per week and the resultant amount of high pH water generated. Sites receiving fewer than ten truck loads of concrete per week are categorised as small, whilst those receiving more than 100 loads per week are categorised as very large.
However, the RPS does not address the issue of high pH water generated from washing other equipment used to handle wet concrete, such as concrete pumps and batch plant. Consequently, categorising a site solely on the number of mixer truck movements can significantly underestimate the risk posed to the surrounding environment. To counter this, Dr Coulton suggests categorising each site on the total volume of high pH water generated, rather than the number of concrete trucks movement. On this basis, a site generating less than 200 litres per week of high pH wash water would be classified as small, whereas one generating in excess of 2,000 litres per week would be deemed to be very large.
He says: “In line with current best environmental practice, the RPS suggests that wherever possible the wash water is reused, and recommends that where this is not possible the preferred disposal route should be to foul sewer, with disposal to ground or surface water being used as a last resort. This is sensible practice as disposal to foul sewer provides the least environmental risk. However, disposal to sewer is covered by the Trade Effluent Regulations and will require the contractor to seek prior approval from the sewerage provider (typically the local water utility company).
“Utility companies are extremely concerned about the discharge of cement and concrete particles into their sewers (because of the risk of blockages) and also require the pH of the water to be less than 10. To comply with these requirements, it is essential that water is appropriately treated prior to discharge into sewer. Unfortunately, simply settling out the solids in a skip or lined pit will not provide sufficient pre-treatment as the pH of the wash water is likely to be in excess of 12. Consequently the concrete wash water will also need to be pH adjusted.
“Despite the old adage that ‘dilution is not the solution to pollution’, it is still a common misconception on many sites that the pH can be reduced simply by flushing the concrete wash water down the sewer together with copious amounts of mains water. Whilst this is theoretically correct, it is both expensive and impractical to achieve.”
Dr Coulton adds it is important to confirm that the discharge point is connected to the foul sewerage network and not a storm water drain, which simply discharges to the nearest surface water course.
Discharge to ground without treatment is only permissible for sites generating less than 200 litres per week of wash water. Importantly, the discharge point must not be within a source protection zone, within 50 metres of a well/borehole or within 10 metres of a water course. It is also necessary to ensure the ground is sufficiently permeable to allow the wash water to soak in (and not run-off directly) into the nearest watercourse, advises Dr Coulton.
“All discharges to the environment are covered by the control of pollution legislation and may need an environmental permit, depending on the size of the concreting operations. Discharges to ground or surface water from small to medium sites do not require environmental permitting by the EA, provided they meet certain criteria and that an appropriate risk assessment has been carried out to demonstrate that pollution will not occur,” he said.
Where concreting operations, producing less than 1,000 litres of wash water, extend longer than 12 consecutive weeks (or involve an intermittent discharge over a longer period), advice needs to be sought from the EA regarding permitting. Sites generating more than 1,000 litres of high pH water per week may require an environmental permit prior to commencing discharge to either surface or ground water. Those generating more than 2,000 litres per week will require both a bespoke environmental permit and a full treatment system before any treated concrete wash water can be discharged into the environment. However, given that the permitting process will take at least 12 weeks, contractors operating larger sites may prefer to seek approval from their water utility company to discharge the water to sewer or simply tanker it off site.
“The underlying message is clear,” says Dr Coulton. “The EA now recognises that the uncontrolled release of high pH concrete wash water can be a major hazard to the environment and must be treated as such. The old practice of just storing the solids in plastic lined skips and “loosing” the wash water on site is no longer acceptable. At the very least, contractors must now undertake a risk assessment prior to discharging even small volumes of high pH water to ground. For most operations, contractors will have little choice but to fully treat the water prior to discharge to either sewer or to the environment. Failure to do so could cause a pollution incident that may well result in prosecution.”