Trade Waste, On-Site Wastewater and Special Drainage Systems
Trade waste
Discharges other than domestic sewage are controlled by the network operator's trade waste bylaw and a discharge consent or permit. The permit sets volume, flow rate, temperature, pH, contaminant limits, pre-treatment requirements, sampling point location and monitoring obligations. Typical pre-treatment includes grease interceptors for food premises, oil and grit separators for vehicle workshops, neutralising or settling pits for acidic or solids-bearing waste, and cooling before discharge so hot water does not damage the pipe or the treatment plant. The cardinal rules: stormwater never enters the sewer, and sewage never enters the stormwater. Sampling points must be accessible and represent the discharge.
On-site wastewater
Where no reticulated sewer is available, design follows AS/NZS 1547 and the G13 Acceptable Solution for on-site wastewater management. Systems range from septic tanks with land application, through aerated wastewater treatment systems and sand or media filters, to composting toilets. Design depends on: occupancy and daily flow, soil category and permeability, depth to groundwater or rock, slope, and setback distances from boundaries, dwellings, waterways and bores. The land application area is sized from the design flow divided by the permitted loading rate for the soil category, and it must be protected from vehicles, stock and roof water. Secondary systems carry maintenance and servicing obligations that must be handed to the owner.
Special systems
Pump stations and rising mains, vacuum drainage, siphonic roof drainage (AS/NZS 3500.3), greywater diversion and backflow prevention all need manufacturer-specific design and commissioning.
Traps
Assuming any drain can take trade waste without a permit. Sizing a grease trap by guesswork. Designing an on-site system without a percolation/soil assessment. Forgetting setbacks.
Rule of thumb: if the discharge is not toilet, bathroom or kitchen domestic flow, ask for the trade waste consent before you design the drain.