Page 5 - Guide to Managing Trade Effluent - 2023

Trade Effluent Consent Conditions

As part of regulating the quality of the effluent which may be discharged into the foul sewer,
limiting values for a range of parameters will be set. Commonly these will include limits on:

• Flow Rate
• Total Daily Volume Chemical Oxygen Demand (COD)
• Time of Discharge
• Temperature Chemical oxygen demand (COD) is a
• pH measure of the capacity of water to
• Toxic Metals consume oxygen during the decomposition
• Suspended Solids of organic matter and the oxidation of
• Sampling and Monitoring inorganic chemicals such as Ammonia and
• Chemical Oxygen Demand (COD) nitrite.

Flow Rate: Limits on the flow rate that can be discharged into the foul sewer are required
to ensure that the sewer system is not overwhelmed. Many foul sewers under specific storm
conditions may discharge untreated sewage to discharge into rivers. Due to the high dilution rates
associated with the storm conditions, this approach is considered acceptable. The flow rate limits
imposed on the flows that may be discharged into a sewer are put in place to ensure that during
The majority of Trade Effluents can be characterised by having the following composition. non-storm conditions when the receiving water body may due to low flow conditions have only
limited ability to dilute the sewage that the sewer is not overwhelmed and does not divert sewage
A liquid phase: The majority of the effluent will comprise a liquid phase. The liquid phase will into the river at a time when the river may have an insufficient level of flow to allow the dilution of
predominantly have been generated through the addition of potable water, either associated the sewage to an acceptable level.
with the production process or via the cleaning/washing down of plant and equipment. The sites
potable water supply will be metered and the typically the water present within the discharge will
have been purchased.

Suspended Solids: Suspended particles may be present within the effluent. These may,
depending on the process being undertaken, be either of an organic biodegradable nature
eg food by- products or may be of a non-degradable nature, eg microplastics, silt or sand. The
Sewage Undertaker will place strict limits on the concentration of suspended particles present
within the effluent as there presence may cause his infrastructure to become blocked and/or they
may be difficult for his sewage treatment works to treat.

Dissolved In-organic Chemicals: Effluent may contain a variety of dissolved inorganic
components eg Calcium, Sulphate, cyanide, Zootoxic Metals eg lead, and/or phytotoxic metals
eg Copper. These components may chemically attack the sewer eg sulphate attach, be toxic
in respect of the microbes used within the sewage treatment works eg cyanide, or may even Rapidly Settling Solids: The
concentrate within the sludge produced at the sewage treatment works, making it unsuitable for concentration of solids present (mg/l)
re-use eg heavy metals. who settling velocity (m/hr) is greater
than 2 m/hr.
Other: Depending on the nature of the process producing the effluent a range of other
compounds may be present. Often whilst these compounds are present in small concentrations, Poorly Settling Solids: The
they may be highly toxic or resilient to treatment at the sewage treatment works. The allowable concentration of solids present (mg/l)
(permitted) concentration within the effluent will be very small. whose settling velocity is < 1 m/hr.

The Sewage Undertaker, will regulate the allowable composition of the effluent that is discharged Total Suspended Solids (TSS): The total
into the sewer. Pre-treatment of the effluent to bring it in line with the conditions stipulated within concentration (mg/l) of solids present.
the Trade Effluent Consent may be required.

4 5