Café grease trap and stormwater discharge proposal
A 140-seat café is fitting out in an existing building. The only drainage connection is a 100 mm foul drain to the public main; the council trade waste bylaw requires pre-treatment for FOG and a sampling point, with annual desludging. The tenant proposes a 35 L grease trap under the sink and discharging the rest to stormwater because "it's only dishwater". There is no room in the tenancy for a large interceptor; a shared alcove in the rear service court is available.
1. Assess the tenant's proposal against the trade waste and drainage requirements.
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The proposal fails on two counts.
First, discharging commercial kitchen wastewater to stormwater is not permitted. Trade waste must go to the foul sewer via the approved pre-treatment system; stormwater is for unpolluted roof and surface water only. Connecting kitchen waste to stormwater is a prohibited cross-connection and a pollution risk.
Second, a 35 L under-sink trap is domestic-scale and is not sized for a 140-seat café. Grease interceptor sizing must follow the council trade waste code and AS/NZS 3500.2, based on the fixtures, flow and FOG load - not on the space available. An undersized trap will pass grease to the sewer, cause blockages and breach the trade waste bylaw.
A trade waste consent/approval is required before connection, and the pre-treatment and sampling point must be shown on the drainage plan.
2. Propose a compliant pre-treatment and connection arrangement using the available space.
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Use the rear service court for a properly sized external grease interceptor, subject to council approval:
- size the interceptor from the café's fixture load and the council trade waste code, not from the tenancy footprint;
- run the kitchen waste in a dedicated gravity line to the interceptor, with adequate grade and access for desludging;
- provide a sampling point downstream of the interceptor and before the connection to the public main, in an accessible position;
- ventilate the interceptor and maintain trap seals; provide a suitable access cover and traffic/load rating if vehicles use the court;
- keep stormwater separate and do not connect any kitchen fixture to it;
- confirm the connection point, levels and capacity of the existing 100 mm foul drain with the territorial authority.
If a shared interceptor is proposed, the ownership, maintenance and cost-sharing arrangements must be documented and accepted by the council.
3. What commissioning and ongoing records should be in place?
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At completion:
- trade waste consent/approval and the approved drainage plan showing the interceptor and sampling point;
- interceptor sizing calculation and manufacturer's installation instructions;
- test results for the kitchen waste line and confirmation that stormwater and foul are separate;
- commissioning record: interceptor filled and checked, sampling point accessible, no leaks, ventilation working;
- as-built levels and locations, plus photographs before covering.
Ongoing:
- a maintenance contract or schedule for desludging at the council-required frequency (at least annually, or more often if the FOG load requires);
- records of every desludge and waste disposal;
- staff instructions on what must not go to the drain (fats, oils, solids);
- the record of work/certification and handover information to the café owner.