Factory process water can enter a sewage holding tank only where the tank, connected drainage arrangements and intended waste handling route are suitable for that particular liquid. A tank designed for welfare sewage should not automatically be used for manufacturing wastewater simply because both are liquids requiring collection.
The deciding factors include what the process water contains, whether it can safely mix with sanitary sewage, the storage system’s suitability and where the collected contents will ultimately go.
Sanitary sewage and process wastewater are different streams
Welfare sewage generally comes from toilets, washrooms and other ordinary sanitary facilities serving employees and visitors. Its source and typical characteristics differ from liquids discharged during manufacturing or industrial cleaning.
Process effluent may originate from equipment washing, rinsing, production lines, cooling operations or other factory activities. Depending on the process, it can contain oils, suspended solids, cleaning chemicals, dissolved substances or altered acidity.
These differences matter even when the wastewater looks relatively clear. Appearance does not establish its chemical composition or whether it is compatible with a sewage holding system.
A factory should establish which drains carry welfare sewage and which receive process water before deciding whether they can share a collection tank.
How mixing changes the contents of a holding tank
When process water enters a tank containing sanitary sewage, the resulting mixture takes on the characteristics of both streams. Its identity cannot be determined from the sanitary component alone.
For example, an incoming stream containing substantial suspended solids may increase sediment accumulation. Greasy wastewater can contribute floating residues, while some cleaning chemicals can alter the chemistry of the combined contents.
Other combinations may create compatibility problems. The relevant question is not simply whether the tank has enough volume, but whether the materials can be stored together under the proposed operating conditions.
Mixing can affect:
- The consistency and composition of sludge left after emptying.
- The suitability of pumps, seals and internal tank materials.
- The information needed to describe collected waste.
- Which receiving facilities can accept the material.
- The frequency of collection as production volumes change.
Dilution with welfare sewage does not remove the need to identify the industrial constituents. Where the stream varies between production runs, information about those variations may be particularly important.
The intended receiving route determines important requirements
A holding tank intended for periodic collection is different from a tank connected to a public foul sewer. That distinction determines which approvals and handling arrangements need consideration.
Collection for off-site management
Where the tank retains wastewater for removal by a waste contractor, its contents must be accurately characterised for collection and acceptance. Industrial components may affect waste classification and the destination suitable for treatment or disposal.
Tank capacity should also be assessed against actual production volumes, peak discharges and available storage between collections.
Discharge to the public sewer
In England and Wales, discharging trade effluent into a public sewer generally requires consent from the relevant sewerage undertaker under the Water Industry Act 1991. Section 118 addresses consent, section 119 covers applications and section 121 provides for discharge conditions. The applicable consent must reflect the nature, composition and quantity of the effluent.
A tank does not remove these obligations simply because the wastewater passes through storage before reaching the sewer. The permitted discharge arrangements must reflect the actual effluent.
Direct discharge to surface water or ground is a separate route. In England, the Environment Agency’s discharge permitting guidance explains when an environmental permit or exemption may apply. This is distinct from both tankered removal and consent to discharge trade effluent into a public sewer; Wales has its own applicable environmental permitting arrangements.
Deciding whether separation is needed
Separating factory drainage streams may be preferable where their compositions, volumes or collection requirements differ significantly. A dedicated process wastewater tank can make it easier to identify contents and manage collection without introducing industrial residues into the welfare sewage system.
On sites where process water must be held temporarily, balance and equalisation tanks illustrate a different storage function from basic sanitary sewage containment. They are not interchangeable simply because both hold wastewater.
Before arranging industrial sewage tank cleaning, managers should establish the tank’s actual contents, whether process streams have entered it and what residues are likely to remain after collection.
The practical answer is that mixed storage may be feasible for a specifically assessed system, but it is not an automatic extension of ordinary sewage storage. Drainage connections, liquid composition, tank compatibility and the receiving route must all support the arrangement.
