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  • Consent conditions

    The aim of trade effluent control is to ensure that discharges either alone or in combination with other effluents or contents of the sewer cannot harm:

    • the sewerage network
    • the sewage treatment works
    • employees and the general public
    • the environment.

    To achieve these objectives conditions are written into consents to control trade effluent discharges on an individual basis.

    Conditions are set to prevent:

    • corrosion of the sewer fabric
    • overloading of sewers and possible flooding of properties
    • blockages of sewers
    • the formation of explosive, flammable and poisonous gases in sewers
    • hazardous situations developing in sewers for maintenance workers such as high temperatures
    • unacceptable discharges from storm water overflows on the sewer network.

    Protection of works

    Conditions are set to protect sewage treatment works and to ensure that:

    • the sewage arriving at the works can be treated effectively and economically
    • damage does not occur to the structure of the works or to mechanical or electrical plant
    • employees operating the works are not harmed
    • biological treatment processes are not affected by toxic effluents
    • the treated effluent produced by the works are suitable for discharge to river
    • the biosolids produced by the works are suitable for disposal to agricultural land.

    Indicative standards to which trade effluent discharges should normally comply with prior to discharge to the public foul sewer in the Wessex Water region are given below. These limits may be varied depending on local circumstances taking into account the volume and rate of discharge, the size and flow of the receiving sewer network and treatment works. 

    Additional requirements may be necessary to control the discharge of toxic and other hazardous substances.

    Limits are normally set as maximum permitted concentration. In certain circumstances limits are applied on the load of a particular substance which can be discharged in a given period (usually 24 hours).

    Ammonia can cause unsafe sewer atmospheres and toxicity problems in watercourses.

    Flammable substances or substances which can produce flammable or explosive atmospheres will be prohibited or controlled to safe levels.

    Fats, oils & greases (FOG) can build up within the sewer which can lead to smell problems, blockages and subsequent foul flooding. FOG can also build up on equipment such as pumps and cause operational difficulties at pumping stations and treatment works 200 milligrams per litre (mg/l) limit is usually applied.

    Free cyanide is toxic and can inhibit treatment processes. The limit is not greater than 3 mg/l. Cyanide is toxic and can inhibit treatment processes.

    Organic load: Chemical oxygen demand (COD) and biological oxygen demand (BOD) may be limited to prevent overloading of the biological wastewater treatment processes.

    Other substances that may be present in the discharge will be assessed and controlled on an individual basis.

    pH extremes can lead to an unsafe working environment, affect biological treatment systems and damage equipment. Low and high pH can result in damage to the materials of construction of the sewer network. The normal range of pH allowed is 6 to 10.

    Prescribed substances are the most dangerous substances to the water environment based on their toxicity and bio-accumulation. The list includes the metals cadmium and mercury, chlorinated solvents and a range of pesticides. Any trade effluent application containing these substances above background levels must be referred to the Environment Agency for determination.

    Sulphide: The limit for substances that can produce hydrogen sulphide upon acidification is normally 2 mg/l. Hydrogen sulphide is a toxic gas that can build up in the sewer atmosphere, leading to hazardous working conditions. Hydrogen sulphide also has a very pungent odour which can cause smell nuisance.

    Sulphate is normally limited to 1000 mg/l because it can cause damage to concrete structures.

    Suspended solids can cause siltation and blockages in the sewerage system. The normal limit is 500 mg/l.

    Temperature is limited to a maximum of 43.3C (110F).

    Toxic metals include chromium, nickel, copper, zinc, lead, silver, tin and molybdenum. Toxic metals can inhibit biological treatment processes and may accumulate in the environment. The Environment Agency regulates the metals content of sewage works discharges and sludge disposal to land.

    Volume: This parameter is normally limited to a number of cubic metres (m3) per 24 hour period and a rate of discharge in litres per second.

    Trade effluent monitoring: We routinely inspect and sample trade effluent discharges to check compliance with consent conditions and for determining the effluent strength for charging purposes.

    Trade effluent sampling is required to:

    • assess trade effluent volumes, strengths and loads
    • provide representative data to enable accurate assessment of trade effluent compliance with consents
    • obtain representative trade effluent charging data
    • assess and control the impact of trade effluent discharges on our wastewater processes.

    The sampling programme is drawn up by the business waste water advisor and subject to a quarterly review.

    Two types of samples will normally be scheduled – charging samples and audit samples.

    Charging samples are used to determine the average discharge strength in order to calculate appropriate trade effluent charges. The appropriate sampling frequency for charging samples is based on:

    • income/chargeable volume
    • variability of quality
    • robustness of sampling method.

    24-hour flow-related composite samples are preferred but if this is not possible or practicable a 24-hour time average composite sample is acceptable. If 24-hour composites are unobtainable or effluent quality is consistent spot samples are acceptable.

    Audit samples are used for monitoring compliance and detecting trends. Sampling frequency for audit samples is based on:

    • potential impact on (and risk to) staff, wastewater systems, receiving watercourses and water supply routes
    • variability of quality and/or flow rate
    • robustness of sampling method.

    Samples scheduled will be a mixture of spot samples for absolute consent conditions and/or composite samples to assess consent load limits. The sampling point and access arrangements must be approved by Wessex Water. An inspection chamber or manhole point is needed so that samples of trade effluent discharge can be obtained for control and charging purposes.

    Sampling points

    Drainage from toilets, sinks, hand-wash basins or showers must not discharge through the point.

    The point must be easily accessed and should not normally be sited in an area with limited access.

    The inspection point must allow samples to be taken without personnel entering a manhole chamber, and without exposing them to any other hazard.

    There should be no obstructions to the inspection point, temporary or permanent. This includes obstruction caused by equipment, materials, deliveries or vehicles. The point should not be locked, and vehicles must not park over it.

    Wessex Water staff should have access at all reasonable times. This is usually within working hours, but in certain circumstances it may be necessary to sample at other times.

    You will be advised by us if space for an automatic sampling device is needed in the manhole. Some companies may wish to make this provision for their own sampling purposes.

    Manhole covers

    To comply with legislation, the manhole cover at a sampling point must be easy to remove. Please note that:

    • a lightweight cover may be required (light covers suitable for heavy traffic are available)
    • if the manhole cover is large, it should be split. Square manhole covers split into two triangles bolted together should be separated by unbolting
    • the keyholes for lifting irons should be positioned to balance the centre of gravity of the manhole. If they are not, the manhole cover can jam on lifting.

    Sampling protocols: It is recommended (especially for high risk discharges) that a trade effluent sampling protocol is agreed. Sampling protocols are designed to ensure that best practice is adopted for trade effluent sampling and monitoring arrangements. This enables us to obtain representative data and samples for the accurate assessment of charges and consent compliance. It also enables the discharger to be assured that robust and clearly understood arrangements exist.

    Health and safety: Both the occupier of the premises and Wessex Water have a duty under the Health and Safety at Work Act 1974 to ensure that employees can safely carry out their duties. Our employees will act with due care for their own health & safety and observe any site specific health and safety arrangements.

    Self-monitoring: You are encouraged to set up self-monitoring programmes if you are a trade effluent customers. This assists with the early detection of problems and is good practice. Self monitoring requirements may be incorporated into trade effluent consents.

    Flow measurement: Consent conditions may require flow measuring equipment to be provided. The cost of installing and maintaining such equipment will be borne by the discharger. Assessment of flow by other means, eg, water meters, process meters or pumping rates may also be considered in some circumstances.

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