Submersible vs. Dry Pit Pumps: Pump Selection Guide for Wastewater

Introduction

Every wastewater collection system includes lift stations — pumping stations that move sewage from low points to higher elevations in the gravity collection system, or from service areas to treatment plants. The central decision in every lift station design is pump type: submersible or dry pit.

Both technologies have been used for decades. Both are reliable when properly selected, installed, and maintained. The choice between them is driven by site conditions, flow rates, maintenance philosophy, and budget. This guide gives you the technical framework to make the right call.

Submersible Pumps

Submersible lift station pumps are installed submerged in the wet well — the pump, motor, and shaft seal are all underwater. The motor is designed to operate in liquid and is cooled by the surrounding wastewater.

How They Work

Submersible pumps are typically centrifugal pumps with semi-open or enclosed impellers. The motor is hermetically sealed and attached directly to the pump volute (no external shaft). They are suspended by their discharge pipe or guide rails and can be removed from the wet well without dewatering it.

Advantages

• Lower capital cost — no need for a dry pit structure, ventilation, access stairs
• Smaller footprint — fits in a simple wet well
• Easy installation — guide rail systems allow pump removal without entering the wet well
• Lower O&M cost for routine operation — no dry pit heating/ventilation to maintain
• Well-suited for remote/unattended locations

Disadvantages

• Motor replacement requires pulling the entire pump unit
• Seal inspection requires removing the pump — you can't inspect the mechanical seal without dewatering or pump removal
• Corrosive environment — the motor operates in harsh wastewater, even with stainless construction
• Limited to moderate flows — very large submersibles (>500 HP) are available but expensive; dry pit is more practical at high flow
• Heat rejection limited by submersion — large motors may overheat in warm wastewater

Common Applications

• Municipal lift stations from 0.1 MGD to 5+ MGD
• Remote manholes and low-flow pump-to-main applications
• Residential/small commercial force mains
• Grinder pump systems (low-flow, high-head)

Leading Submersible Pump Manufacturers

• Ebara — stainless and cast-iron submersible and dry-pit water/wastewater pumps
• DanPumps — S-WP submersible and S-WN dry-installed wastewater pumps
• Landia — submersible and dry-installed chopper pumps for rag/solids handling
• Flygt (Xylem) — market leader, N-pump (non-clog) technology
• Grundfos — SE/SL series submersibles
• Wilo — submersible sewage pumps
• Sulzer (ABS) — XFP submersible series
• Cornell Pump — cast iron submersibles for municipal lift stations
• Gorman-Rupp — submersible series

Dry Pit Pumps

Dry pit pumps are installed in a separate dry chamber below grade (or at grade), with only the suction inlet submerged or connected to the wet well. The pump and motor are accessible in a dry, ventilated environment.

How They Work

Dry pit configurations come in several sub-types:

• Vertical turbine (column) pumps — motor at top, long shaft down to impeller below wet well floor. Traditional design.
• Horizontal split-case pumps — motor and pump at same elevation, connected by shaft and flexible coupling. Common for large stations.
• End-suction horizontal — compact, less common in wastewater.

The pump shaft passes through the wet well floor via a packing gland or mechanical seal, and the motor is entirely out of the water.

Advantages

• Full accessibility — mechanics can see, inspect, and maintain the entire pump without removing it from the well
• Better seal inspection — mechanical seals are visible and inspectable
• Lower pump replacement cost — motor and pump can be serviced or replaced separately
• No size limitation — dry pit is practical for pumps of any size, up to thousands of HP
• Longer service life — in dry, controlled environment, motors last longer
• More efficient at large flows — horizontal pumps have better hydraulic efficiency curves at high flow/low head

Disadvantages

• Higher capital cost — dry pit structure, ventilation, lighting, sump pumping, stairway access
• More complexity — shaft seals can leak; dry pit flooding is a failure mode
• Larger site footprint — requires the dry pit structure in addition to wet well
• Maintenance risk — mechanics must enter a confined space to service the pump

Common Applications

• Large municipal lift stations (>5 MGD)
• Critical pump stations where reliability and maintainability justify higher capital cost
• Stations with >24/7 operator presence
• High-head applications (>100 ft TDH) where vertical turbine efficiency is advantageous

Leading Dry Pit Pump Manufacturers

• Ebara — stainless and cast-iron submersible and dry-pit water/wastewater pumps
• DanPumps — S-WN dry-installed and S-WP submersible wastewater pumps
• Landia — submersible and dry-installed chopper pumps for rag/solids handling
• Gorman-Rupp — self-priming T-Series and packaged above-grade lift stations
• Flygt (Xylem) — dry-installed submersible/non-clog pumps
• Xylem (A-C Pump) — large vertical and horizontal split-case
• Flowserve — large horizontal split-case for major pump stations
• BBA Pumps — self-priming and dry pit for municipal
• Smith & Loveless — packaged lift stations with horizontal dry pit pumps

Decision Matrix: Submersible vs. Dry Pit

Special Pump Types for Wastewater

Grinder Pumps

Submersible pumps with cutting mechanisms that shred solids before pumping. Used in low-pressure sewer systems serving individual homes or for pumping raw sewage with large solids. Flow: 0.5-5 GPM typical.

Chopper Pumps (Vaughan)

Large submersible or dry pit pumps with a chopper impeller that cuts and shreds rags, wipes, and solids as they enter. Critical for systems with high rag loads. Vaughan Company is the dominant manufacturer.

Progressive Cavity Pumps

Used for high-viscosity or high-solids applications — sludge pumping, chemical dosing. Not common for lift station service.

Self-Priming Centrifugal (Gorman-Rupp T-Series)

A unique configuration: above-grade pump that can self-prime, drawing wastewater up from below. No wet well required for installation in some configurations. Popular for remote pump stations and temporary dewatering.

Common Lift Station Failures to Watch

• Rag buildup on impeller — clog and cavitation damage. Wipes and non-flushables are the primary cause. Chopper or cutter pumps help.
• Bearing failure — typically from running dry or water intrusion
• Seal failure — submersible seal failure allows water into motor; dry pit seal failure allows sewage into dry pit
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