2.4 Pumped Drainage Systems, Sumps & Rising Mains

Design note: Storage, pumps, controls, alarms, rising-main diameter and velocity, valves, and discharge arrangement are outputs of the approved system and network design. Figures in a worked calculation are example inputs, not national minima.

Pumped Drainage Systems, Sumps & Rising Mains

Gravity drainage is the foundational principle of sanitary engineering. However, in modern construction, gravity discharge to the municipal sewer is not always topographically possible. Basements, subterranean carparks, low-lying coastal subdivisions, deep hillside excavations, and properties located below the level of the public street sewer necessitate the collection and mechanical lifting of wastewater. These installations are known as pumped drainage systems.

Because mechanical pump failure or power blackouts can rapidly result in catastrophic internal flooding with raw sewage, the design, sizing, and installation of wet wells, pumps, rising mains, and valves are subject to rigorous technical standards under AS/NZS 3500.2:2021 Section 10 and NZBC Acceptable Solution G13/AS2 Clause 6.0.

---

1. Wet Well / Collection Sump Engineering

A sanitary drainage wet well (pumping sump) collects wastewater by gravity from low-level fixtures and stores it temporarily until a predetermined operational volume triggers the pump to run.

Sump Construction & Durability

• Watertight Structural Integrity: Wet wells must be completely impermeable to prevent foul water from exfiltrating into the surrounding subsoil or groundwater from infiltrating into the sump. Approved materials include precast reinforced concrete with internal epoxy coatings, rotomoulded heavy-duty polyethylene (PE), or glass-reinforced plastic (GRP) manufactured to AS/NZS 1546.1.
• Hydrogen Sulfide (H2S) Corrosion Protection: When wastewater remains stagnant in a sump, anaerobic bacteria convert sulfates into hydrogen sulfide gas. Aerobic bacteria (Thiobacillus) on the moist walls above the water line oxidize this gas into biogenic sulfuric acid (H2SO4). Unprotected concrete sumps can suffer structural failure within 5 to 10 years; epoxy resin linings or HDPE interior liners are mandatory.
• Benching and Sump Geometry: Flat-bottomed sumps are strictly prohibited. The bottom of the wet well must be finished with steep self-cleansing benching or hopper slopes angled at not less than 1:2 (45 degrees to the horizontal) toward the pump suction intake. This prevents heavy organic solids from depositing in dead corners, decomposing, and releasing toxic gases.
• Sealing & Venting: The wet well cover must be gas-tight and bolted securely with an elastomeric gasket. The chamber must be vented to open atmosphere above roof level via a dedicated DN50 to DN100 vent pipe.

---

2. Storage Volume & The 24-Hour Emergency Reserve Rule

A sanitary wet well comprises three distinct functional volumes:

1. Dead Storage Zone: The permanent water level below the pump cut-out switch. Submersible pump motors rely on surrounding wastewater for thermal cooling. The pump must shut off before the motor housing is exposed to air or vortexing occurs.
2. Operational Cycling Zone: The volume between the pump cut-in level and cut-out level. This volume must be sized in conjunction with pump flow rate to prevent excessive motor cycling. Motors should not cycle more than 10 to 15 starts per hour to prevent thermal overload.
3. Emergency Storage Zone: The volumetric capacity located between the high-water alarm level and the invert of the lowest incoming gravity drain.

The Mandatory 24-Hour Residential Rule (AS/NZS 3500.2 Clause 10.5)

For domestic installations, if the municipal power grid fails or the pump trips on overload, occupants must still be able to use sanitary facilities without sewage backing up out of basement shower wastes or floor drains.

Statutory Requirement: In domestic residential installations, the emergency storage capacity between the high-water alarm float and the lowest incoming drain invert must match the approved design basis and the network utility operator’s requirements.

• Design Flow Allowance: Standard NZ sizing uses 150 to 200 Litres per person per day.
• For a typical 4-person residential dwelling:
  • Emergency Storage Volume = 4 * 200 L = 800 Litres
• In commercial, multi-unit residential, or critical health facilities, emergency storage may be reduced to 4 to 8 hours only if the site is equipped with dual duty/standby pumps connected to an automated emergency backup diesel generator.

Audio-Visual Telemetry Alarm

Every pump station must have a high-water alarm activated by an independent level float located at the bottom of the emergency storage zone. The alarm must feature an external flashing beacon and an audible buzzer placed in an occupied area, powered by an independent battery backup circuit.

---

3. Pump Selection: Grinder vs Vortex Pumps

Dual Duty/Standby Configurations

For commercial, public, or multi-dwelling installations, a single pump is strictly unacceptable. The station must be fitted with dual identical pumps operating in a duty/standby arrangement:

• Lead/Lag Alternation: An automated control panel alternates the duty pump on each cycle to equalize motor wear.
• Automatic Failover: If the duty pump fails to start or the inflow exceeds a single pump's capacity, the control panel sounds an alarm and automatically starts the standby pump.
• Guide Rail Auto-Coupling: Submersible pumps must be mounted on stainless steel guide rails with an auto-coupling duckfoot base at the sump floor. This allows pumps to be hoisted out for servicing from ground level using lifting chains without anyone entering the confined, hazardous wet well.

---

4. Rising Main Engineering: Velocity & Materials

A rising main is a closed conduit under positive hydraulic pressure discharging pumped wastewater up to a gravity sewer. Sizing a rising main requires balancing two opposing hydraulic constraints:

Flow Velocity V = Q / A = (4 * Q) / (pi * D_i^2)

Where Q is pump discharge flow rate (m3/s) and D_i is pipe internal diameter (m).

The Velocity Envelope (AS/NZS 3500.2 Clause 10.7)

1. Minimum Cleansing Velocity (0.75 m/s): If velocity falls below 0.75 m/s, heavy organic solids, greases, and grit drop out of suspension, settling in low points of the rising main. Over successive cycles, these solids compact, choking the pipe bore and causing severe blockages.
2. Maximum Velocity (1.8 m/s): If velocity exceeds 1.8 m/s, dynamic friction losses escalate dramatically, overworking the pump motor. More critically, high velocities generate severe water hammer (transient pressure surges) when the pump stops and the check valve snaps shut, which can rupture pipe fittings.

Permissible Velocity Range: 0.75 m/s <= V_rising main <= 1.80 m/s

Pipe Material Selection

• PERMITTED PRESSURE MATERIALS:
  • PE100 (HDPE): Minimum PN10 (SDR17) or PN16 (SDR11) with electrofusion or butt-welded joints. Flexible, resistant to ground settlement and H2S attack.
  • PVC-U Pressure Pipe: Minimum PN9 or PN12 solvent-cemented or rubber-ring jointed pressure pipe.
  • Ductile Iron (DI): Pressure-rated with polyurethane or polymeric internal lining.
• STRICTLY PROHIBITED:
  • Non-Pressure DWV / SN4 uPVC Pipe: Thin-walled drain-waste-vent pipe is engineered for gravity flow only. Under cyclic pressure shockwaves from pump startups and valve closures, DWV solvent joints and fittings will fracture, saturating the ground with raw sewage.

---

5. Valve Assemblies & Gravity Discharge Interface

Every pumped drainage installation requires specific valving arranged in a dedicated, accessible valve chamber adjacent to the wet well:

• Non-Return Valve (Check Valve): A full-bore swing check valve or resilient-seated ball check valve must be installed on the discharge line of each pump. When the pump shuts down, the check valve prevents the entire volume of wastewater stored in the rising main from draining back into the wet well, which would cause short-cycling and motor burnout.
• Isolating Valve: A resilient-seated wedge gate valve or full-bore ball valve must be placed immediately downstream of the check valve. This allows the check valve and pump to be isolated and removed for maintenance without draining the rising main.

Termination at the Gravity Sewer: The Air Break Rule

A rising main must never connect directly into a standard gravity drain junction or gully trap. Discharging high-pressure sewage directly into a gravity pipe causes extreme hydraulic turbulence, destroys open-channel flow, strips fixture trap seals, and blasts sewer gas through gully grates.

Discharge Interface Requirement: The rising main must terminate at a discharge manhole or break tank, discharging through an air break (minimum 100 mm above the invert) against an energy-dissipating baffle or smoothly into the receiving channel haunching.

---

6. Worked Sizing Calculation: Residential Basement Pump Station

Design Data

• Occupancy: 6-person large basement residence.
• Inflow Rate: Daily Inflow = 6 persons * 200 L/day = 1,200 L/day.
• Static Lift: Height from sump floor to discharge manhole = 4.5 m.
• Rising Main Developed Length: 32.0 m.
• Pump Selection: Single residential grinder pump delivering Q_pump = 2.0 L/s (0.002 m3/s).

Step 1: Emergency Storage Tank Sizing

• Required emergency storage volume = 24 hours * 1,200 L/day = 1,200 Litres.
• Assuming a cylindrical wet well with internal diameter D = 1.20 m:
  • Cross-Sectional Area A = (pi * 1.20^2) / 4 = 1.131 m2
• Required height for emergency reserve:
  • Height = 1.200 m3 / 1.131 m2 = 1.06 m
• Allowing 0.40 m for operational cycling and 0.35 m for bottom dead storage/benching, the wet well must have a total depth of at least 1.81 m (1,850 mm) below the incoming gravity invert.

Step 2: Rising Main Pipe Diameter Selection

Test two potential pipe sizes for a flow rate of 2.0 L/s (0.002 m3/s):

Option A: DN40 PE100 SDR11 (Internal Diameter D_i = 32.6 mm = 0.0326 m)

• Pipe area A = (pi * 0.0326^2) / 4 = 0.000835 m2
• Flow velocity V = 0.002 / 0.000835 = 2.40 m/s
• Evaluation: 2.40 m/s exceeds 1.80 m/s. Velocity is too high, creating excessive dynamic head loss and severe water hammer risk. DN40 is non-compliant.

Option B: DN50 PE100 SDR11 (Internal Diameter D_i = 40.8 mm = 0.0408 m)

• Pipe area A = (pi * 0.0408^2) / 4 = 0.001307 m2
• Flow velocity V = 0.002 / 0.001307 = 1.53 m/s
• Evaluation: 1.53 m/s falls squarely within the mandatory 0.75 m/s to 1.80 m/s envelope. DN50 PE100 is selected for the installation.

---

7. Trade Traps & Common Installation Pitfalls

• The DWV Rising Main Blunder: Installing standard SN4 DWV uPVC with standard clear solvent cement for a rising main to save money. The first time the pump shuts down against a high static head, the hydraulic pressure spike cracks the 90-degree bend socket, flooding the subfloor with raw sewage.
• Omission of the Swing Check Valve: Omitting the non-return valve or installing it backward. When the pump stops, the entire volume of sewage inside the 30 m rising main drains back into the wet well, raising the level and restarting the pump. The pump cycles continuously every 45 seconds until the motor burns out.
• Direct Injection into Gravity Pipes: Connecting a pumped rising main directly into a branch drain using an oblique junction. The pressurized jet blasts straight up into domestic fixture traps and strips all water seals in adjacent bathrooms. Always terminate in an engineered discharge manhole with an air break.