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This paper considers the practical aspects of treating wash water from concreting operations, both on site and at batching plants, to a standard suitable for disposal to either sewers or surface and groundwater.
In particular, it considers what level of treatment is required to ensure compliance with either a trade effluent consent, for discharge to sewers, or an environmental permit, for discharge into the aquatic environment be it surface water or groundwater, and lastly how this can be cost effectively and reliably achieved.
For England and Wales the required treated water standard is set by either:
• The water utility company for discharge to sewer – This is enforced by a trade effluent consent, issued by the water utility company in accordance with the Water Act 1990
• The Environment Agency / Natural Resources Wales for discharge to surface or ground water – This is enforced by an environmental permit issued in accordance with the Environmental Permitting Regulations (England and Wales) 2010
The water quality standard required by a water utility company, for discharge to foul sewer, will be determined by the flow rate (both instantaneous and maximum 24-hour daily) and pollution load that the receiving waste water treatment works (WWTW) can handle. In instances where the receiving WWTW has no spare capacity, the utility company has the right to refuse the water, which in some instances may necessitate taking it away in tankers as liquid waste at huge cost.
The standard set by the Environment Agency (England) and Natural Resources Wales (Wales) will be based on the ability of the receiving watercourse to accommodate any additional pollutant load without causing any significant deterioration in water quality.
Whether you are discharging to a foul sewer or into the aquatic environment, the maximum daily and instantaneous flows will be limited, together with limits on the other key pollutants that affect water quality.
As such, the water quality treatment standard needed to be achieved is location specific and will vary depending on the available capacity of the receiving water course or waste water treatment plant. In the case of discharge to the aquatic environment, the limits will depend not only on whether the discharge is to surface, ground or coastal waters, but also the environmental status of these waters. For example, a tighter standard may be imposed on rivers supporting salmonoid fish or coastal waters supporting shellfish or other protected species, such as fresh water pearl mussels. Typical values are summarised in Table 1 and in addition to the obvious limits on suspended solids and pH, limits may be imposed on the concentrations of sulphate, chloride, chemical oxygen demand (COD), biological oxygen demand (BOD), chromium, and other contaminants found in treated concrete wash water.
Both disposal routes involve cost not only in terms of on-site treatment, but also in disposal charges. The cost of disposal to sewers is determined by the Mogden formula, which quantifies a unit treatment charge in terms of the cost of handling and treating the wash water at the receiving water treatment works. The cost of direct disposal to the environment under an environmental permit is determined on the basis of the volume of water being discharged and the perceived risk to the environment in accordance with a formula that takes into account the scale and nature of the activity and the applicant’s track record.
The most common treatment requirement is the removal of suspended solids and the reduction of pH to an acceptable value. The selection of method for suspended solids removal is relatively straight forward, typically involving settlement of the solids with or without the benefit of flocculant and/or coagulants. The selection of the most appropriate method of pH adjustment, however, is a little more involved. It needs to take a number of factors into consideration:
• The health, safety and environmental issues associated with the reagents used for pH adjustment
• The ease of control and in particular the risk of overdosing – acidifying the water
• The limits imposed by the trade effluent consent or environmental permit on secondary pollutants added by the pH adjustment process – i.e. chloride, sulphate, chemical/ biological oxygen demand
• Cost
Concrete wash water typically has a pH of between 11 and 13.6 (depending on the degree of dilution) due to the presence of dissolved lime (calcium hydroxide) and other hydroxides released as the cementitious material hydrates. Similarly, slightly less alkaline waters (typically pH 10 to 12) are generated by hydro-demolition works and rainfall either running off freshly poured concrete slabs or percolating through hard standing areas built from steel slag or recently crushed concrete.
As pH is a log scale, the use of dilution to reduce the pH is neither practical nor cost effective, as every unit reduction in pH requires a 10 fold dilution with pH neutral water.
For a typical concrete wash water with a pH of 12 the following dilution volumes would be required:
pH 11.7 1:1
pH 11 10:1
pH 10 100:1 Typical upper limit for discharge to sewer
pH 9 1000:1 Typical upper limit for discharge to the environment
pH 8 10,000:1
Consequently even small volumes of concrete wash water have the potential to have a significant impact when discharged into the aquatic environment.
The most commonly used reagents are mineral acid (either sulphuric or hydrochloric acid), citric acid and carbon dioxide.
Traditionally pH adjustment has been undertaken using mineral acid, but this has a number of disadvantages, namely:
1. Both sulphuric acid and hydrochloric acid are dangerous to handle and need to be stored in a secure bunded enclosure, to minimise the risk to operatives and accidental release into the environment.
2. Precise pH control is difficult to achieve due to near vertical gradient of the titration curve over the normal treated water pH range, see Figure 1. Due to the logarithmic nature of the pH relationship, in excess of 90% of the acid is consumed in reducing the pH from the initial value to around 11 and only a few per cent is consumed in further reducing the pH to within the target range. As shown in Figure 1, the addition of a very small excess of acid results in the pH dropping below the target range and the discharge of acidic water off site. To achieve accurate pH control, the acid needs to be added in a number of stages and a failsafe system installed to prevent the treated water being discharged off site if out of specification (over dosed).
3. The addition of sulphuric acid increases the sulphate concentration of the treated water, and similarly the addition of hydrochloric acid results in an increase in the chloride concentration in the treated water. Both of these are considered to be pollutants in their own right and as shown in Table 2 the concentration of these ‘secondary pollutants’ following pH correction of concrete wash water with mineral acid is likely to exceed both the trade effluent and/or the discharge permit consent limits. Consequently, while the careful controlled use of mineral acid will bring the pH to within the consented range, the treated water will still be out of specification due to the presence of the ‘secondary pollutants’ introduced by correcting the pH.
4. Specialist handling, training and PPE are required for operatives handling the acid on site.
5. Any surplus unused acid will need to be disposed of, at considerable cost, as hazardous waste at the end of the project.
6. As shown in Table 3, both sulphuric and hydrochloric acid is circa four times more expensive than using carbon dioxide with a typical chemical cost of around £1.50/m 3 .
Citric acid (fruit acid) can be used as an alternative to mineral acid and is typically added either as a pre-prepared solution or as granules, which slow dissolve in the water. Although the granular form is easier to handle, the use of citric acid suffers from the following disadvantages:
• Similar environmental, safety and health concerns are associated with the use of citric acid, due to its relatively low pH when dissolved in water
• As per mineral acid there is a risk of over dosing and acidifying the treated water (see Figure 2), particularly when used in the solid form, as the pH continues to decline while the granules continue
• The addition of the citric acid will increase the biochemical oxygen demand of the water above the limits for safe discharge into the aquatic environment (Table 2)
• In terms of cost, citric acid is far more expensive than any other form of pH adjustment (Table 3), with a typical chemical cost of circa £2.50/m 3
• Theoretically any excess citric acid should be disposed of as hazardous waste
Mildly acidic when dissolved in water, carbon dioxide has become the neutralising agent of first choice as it overcomes a number of difficulties associated with the use of either citric acid and/or mineral acid:
• Carbon dioxide is self-buffering, as it is virtually impossible to acidify the water though overdosing (Figure 3)
• Unlike citric and mineral acids, the by-products of neutralising the water with carbon dioxide are non-hazardous and in certain circumstances are considered to be beneficial (see Table 1)
• The gas is easy to store and easy to dose, overcoming the safety, health and environmental concerns associated with the use of either mineral acid or citric acid
• Partly full cylinders can simply be returned to the gas supply, without incurring any additional cost associated with the disposal of hazardous waste
• The use of carbon dioxide increases the alkalinity (hardness) of the water, which for a number of pollutants reduces their impact on the environment e.g. chromium (see Table 1)
• As shown in Table 3, carbon dioxide is the most cost effective treatment option, with an actual treatment cost of around £0.35/m 3
Summary
After considering the health and safety, technical, environmental, and cost issues, it is easy to understand why the construction industry has concluded that the use of carbon dioxide to neutralise highly polluting concrete wash water is their preferred solution.
Published: 04th Mar 2015 in AWE International
