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Wastewater is produced from various industrial processes such as mining, power generation, petroleum refineries, breweries, dairy processing, chemical production, food production and many more.
Most industries are now required to treat their wastewater on site before it is discharged to river or sewer (to meet a discharge consent limit), but this treatment will vary depending on the contaminants present. Domestic wastewater will include both sewage and ‘grey’ water, such as outflow from baths and washing machines, and underground pipes will collect the wastewater, which ends up at a centralised urban wastewater treatment works (UWwTW).
Wastewater can be defined as water which is of no further immediate value to the purpose for which it was used or in the pursuit of which it was produced, because of its quality. However, it is possible that wastewater from one producer can be used as a potential supply to another organisation. Typical methods of treatment include chemical immobilisation, neutralisation and precipitation. Three broad types of treatment are distinguished: primary, secondary and tertiary.
Primary treatment
Treatment of wastewater by a physical and/or chemical process involving settlement of suspended solids, or other process in which the Biological Oxygen Demand (BOD) of the incoming wastewater is reduced by at least 20% before discharge, and the total suspended solids content is reduced by at least 50%.
Secondary treatment
Post-primary treatment of wastewater by a process generally involving biological or other treatment with a secondary settlement or other process, resulting in a BOD reduction of at least 70% and the Chemical Oxygen Demand (COD) reduction of at least 75%.
Tertiary treatment of public wastewater
Treatment (additional to secondary treatment) of nitrogen and/or phosphorus, and/or any other pollutant affecting the quality of a specific use of water, e.g. microbiological pollution, colour etc.
For organic pollution, the treatment efficiencies that define a tertiary treatment are the following: organic pollution reduction of at least 95% for BOD, and 85% for COD, and at least one of the following:
- Nitrogen reduction of at least 70%
- Phosphorus reduction of at least 80%
- Microbiological reduction achieving a faecal coliform density of < 1000 in 100 ml
In addition, specific industrial effluents may require additional treatment for specific determinands that may not be common in most wastewaters.
In all cases, the treated wastewater will require testing to ensure it is suitable for discharge and meets the requirements of the regulators. The Environment Agency produced an MCERTs standard entitled: Performance Standard for Organisations Undertaking Sampling and Chemical Testing of Water (v.2 2013) and organisations should comply with this standard. MCERTs stands for the Monitoring Certification Scheme and several of these standards were published covering air monitoring, soil analysis, and one for the monitoring equipment itself, which is also relevant to the wastewater industry.
The MCERTs standard
The aim of the MCERTs standard is to provide quality environmental measurements, with the scheme being based on international standards. It provides for the product certification of instruments, the competency certification of personnel, and the accreditation of laboratories. The document details the requirements for sampling and chemical testing of untreated sewage, treated sewage effluents and trade effluents.
Laboratories must be accredited to ISO/IEC 17025 specifically for the sampling and testing of these matrices, and the document covers the following aspects:
- Performance targets
- Selection and validation of test methods
- Pre-treatment and preparation of samples
- Ongoing quality control
- Participation in proficiency testing schemes
- The reporting of test results and other information
- Sampling procedures
The MCERTs standard does not restate all the requirements of ISO/IEC 17025, but concentrates on the additional requirements which must be complied with for an organisation to become MCERTs accredited.
Analytes listed in the MCERTs standard
The range of analytes covered by the MCERTs standard is listed in Annex A of the standard, split into three tables consisting of metals, general physical/inorganic determinands, and organic compounds. Commonly requested determinands include:
- pH
- Electrical conductivity
- Dissolved oxygen
- Arsenic, boron, mercury, selenium, sodium and tin (common in untreated sewage)
- Aluminium, barium, cadmium, chromium, copper, iron, lead, manganese, nickel, vanadium, zinc (potentially toxic metals, although others can be included)
- Magnesium, calcium, sodium, potassium (alkali metals)
- Alkalinity
- Ammoniacal nitrogen
- BOD and COD
- Suspended solids and dissolved solids
- Nitrates, nitrites, phosphates, sulphate and chloride
- Herbicides
- Pesticides
- Volatile organic compounds
- Organotin compounds
- Phenolic compounds
- Polychlorinated biphenyls (PCBs)
- Hydrocarbon oils
- Polyaromatic hydrocarbons (PAHs)
BOD is the Biochemical Oxygen Demand for a period of five days. It is equal to the amount of dissolved oxygen required by organisms for aerobic decomposition of organic matter present in the water. This is measured at 20°C for a period of five days, and provides information on the degree of water pollution with organic matter.
COD is the Chemical Oxygen Demand and is the mass concentration of oxygen equivalent to the amount of a specified oxidant consumed by dissolved or suspended matter when a water sample is treated with that oxidant under defined conditions.
This is not an exhaustive list, particularly with the industrial wastewaters, as these may contain less common analytes such as cyanide, formaldehyde, sulphide, explosives, alcohols, and aldehydes. The tables in Annex A also include the precision and bias targets which the laboratory must meet for the determinand in question for the method to be accredited to MCERTs.
Examples are shown in the table above.
Precision is expressed at percent relative standard deviation.
Bias is expressed in percentage terms.
Generally, the limits for precision and bias are stricter for inorganic and physical determinands as opposed to organic compounds.
Bias – this may be positive or negative, and is the difference (expressed as a percentage) between the mean of a number of determinations and the true or accepted concentration:
Bias can be estimated where appropriate certified reference materials are available and a stated (certified) concentration has been quoted.
Recovery data can be used to estimate bias via spiking experiments (spike and recovery).
Precision – this is the distribution of a number of repeated determinations, obtained under specific conditions, expressed in this standard as the % relative standard deviation (RSD):
Where S is the total standard deviation and M is the mean of the results.
Sampling
The organisation shall have procedures in place to ensure equipment, reagents and samples will be protected from damage or degradation during collection, transportation and subsequent storage prior to analysis.
The sampling management system should include, but not be limited to, the following procedures:
- A sampling programme, including procedures for resampling
- Methodologies for taking samples
- Training and audit records
- Use of appropriate bottles and preservation techniques
- Documentation of sample transport, receipt, handling, storage, retention, delivery and chain of custody
- Records of operation, maintenance and calibration of equipment used in sampling
- Records of operation, maintenance and calibration of on-site test equipment
- Quality assurance procedures for assessing sampling activities
Sample containers generally include the following:
- 1 x litre HDPE bottles for metals and general inorganic/physical determinands
- 1 x litre coloured glass bottles for organic determinands
- 2 x 40 ml amber glass vials with a septum in the lid for VOC analysis
- 100 ml HDPE bottles containing specific preservatives for unstable parameters, e.g. ammoniacal nitrogen, sulphide, cyanide, phenols
If a preservative is used, then this should be recorded on the Chain of Custody document. Similarly, if the sample is filtered on site, this should also be recorded on the C of C. It is crucial that site personnel are informed as to whether dissolved determinands are required (samples are filtered) or total determinands, where any sediment is included in the analysis. This is often important for effluents discharging to controlled waters, where the total sample loading may affect the receiving waters. If filtration is required, it should be performed on site at the point of sampling, and if this is not practicable, this should be reported and justified with the submission of results.
Samples requiring preservation should generally be filtered through 0.45 micron filters prior to filling the bottles containing the preservative. This will not apply to samples requiring organic analysis, as the filters may remove some of the organic determinands, so these should not be filtered.
Samples should be stored in an environment which maintains a temperature of 4.5 +/- 3.5°C, although it is appreciated that it may take some hours for samples to reach this temperature range. At the laboratory, the temperature of the samples is recorded using a calibrated infrared thermometer, which records the surface temperature of the sample vessels in the cool box. Information regarding this temperature is passed on to the client by the laboratory. Storage in the laboratory is maintained at, or close to, 5°C.
Ensuring quality of data
Laboratories should demonstrate that method performance is maintained to the MCERTs standard in a statistically controlled manner, with historical verification of the performance across the range of matrices requiring analysis, and that measurement uncertainties are estimated. This can be accomplished by the following procedures:
• Participation in proficiency testing (PT) programmes, with all data available for review
– PT schemes involve the laboratory paying for scheme membership, whereby samples of known concentration are sent monthly to the laboratory. Results are then reported to the organisers, who will then provide a summary of all data and data scores, which is coded to maintain the confidentiality of the laboratories
• Internal quality control checks for all batches, recorded and audited by UKAS
– These are the standard AQC checks performed per batch (blanks, standards, possibly CRMs), with a typical batch consisting of 20 samples or fewer
• Use of certified reference materials (CRMs) for validation and QC checks
– CRMs are spiked wastewaters and use of these allows the laboratory to ensure its methods are robust and verified for the range of matrices in question
• Spike and recovery, where CRMs of a specific matrix are not available
– Only a limited range of CRM matrices are currently commercially available, and therefore the laboratory should add known concentrations of determinands to a typical matrix to prove the method is effective on a specific type of matrix
All validation data, spiking data and proficiency testing data will be reviewed by UKAS during audits, and in laboratories with MCERTs, this data should also be made available to the Environment Agency, should they choose to request it. In addition, clients may request copies of proficiency testing data.
Summary
Wastewater effluents, if inadequately treated, can result in increased nutrient levels, often leading to algal blooms; depleted dissolved oxygen, resulting in fish kills and loss of other fauna; destruction of aquatic habitats with sedimentation, debris, and increased water flow; and acute or chronic toxicity to aquatic life from chemical contaminants, as well as bioaccumulation of chemicals in the food chain. These effects are economically, socially, and environmentally unsustainable, and therefore all wastewater should be treated prior to discharge. The Environment Agency produced the MCERTs standard to protect the quality of receiving waters from effluent discharges, and a stringent list of requirements to be followed is provided in this standard.
