When pumping or macerators are acceptable, how their discharge pipes are laid out and how basements are protected from sewer surcharge.
When pumping is acceptable
Gravity drainage comes first. A pumping installation is used where gravity is impracticable or where protection from a surcharging sewer is needed. Package units inside buildings should meet BS EN 12050 and be designed to BS EN 12056-4. Approved Document H sizes the effluent receiving chamber for 24 hours of inflow where the building’s foul drainage is pumped, using a minimum domestic allowance of 150 litres per person per day. Where only a proportion is pumped, calculate the storage pro rata. This is receiving-chamber storage, separate from the pump’s flow and head rating.
Macerator units
In England a WC with a macerator and pump may discharge through small-bore pipe to a stack only under two conditions. There must also be access to a WC that drains by gravity, and the unit must meet BS EN 12050-1 or BS EN 12050-3. For pumped washing facilities, Approved Document G paragraph 5.10 specifically cites BS EN 12050-2 and requires access to gravity-drained washing facilities. WC macerators to BS EN 12050-3 are limited-application units: manufacturers require the unit to be in the same room as the appliances it serves and accessible for maintenance. Pipe routing must follow the model’s instructions. For the earlier Saniplus model used in the practice examples, the lift comes first and the following run falls towards the stack at no less than 1 in 100. Its manual calls for smooth bends and a suitable boss connection. When a discharge drops well below that unit and siphons its seal, its specified remedy is a suitable high-point AAV rated to withstand 10 psi. A gravity soil-stack AAV may not suit that pressurised pipe.
Part 1 of BS EN 12050 covers faecal wastewater and Part 2 covers faecal-free wastewater. Part 3 covers limited applications: its 2015 scope includes both, subject to its appliance and installation limits. Keep that current scope separate from the older editions cited in Approved Document G; do not describe Part 3 as exclusively for WC waste.
Sizing the pump and pressure pipe
At the design flow, add the geodetic lift from the switch-off level to the gravity-drain connection to the pressure losses in pipework, fittings and valves. For example, 2.4 m lift plus 0.8 m pipe loss and 0.3 m fitting loss needs 3.5 m head at that flow. Check the selected pump curve and actual operating point.
The cited pumping guide uses a pressure-pipe velocity range of 0.7–2.3 m/s to balance deposition, friction and pressure-transient risks. Route the pressure pipe in a loop above the site’s assessed backflow level before joining gravity drainage. Street level at the sewer connection is the guide’s default if no different local level is defined; it is not an absolute upper limit on flooding.
Protecting against surcharge
For basements with sanitary appliances, the sewerage undertaker’s view of the risk decides the approach: pump where the risk is high, fit an anti-flooding valve where it is low. For other low-lying sites at low risk, the guidance permits an outside gully at least 75 mm below floor level, positioned so any overflow causes no building damage. Anti-flooding valves should suit foul water, preferably use a double-valve arrangement and provide manual closure. One valve should not normally serve multiple buildings. Display a notice describing the protected drainage, override and maintenance. Drainage unaffected by surcharge bypasses the protection and flows by gravity.
Faults to recognise
For the earlier Saniplus model, intermittent starts prompt checks for dripping connected appliances and a faulty non-return valve. A long run after use prompts checks of lift, length, bends and possible chamber blockage. These are fault-table leads, not certain diagnoses from one symptom. Follow the model’s service procedure and safely isolate electricity before working on the unit; dismantling requires the appropriate service agent.