What separates a sanitary manhole specification from a storm manhole specification written for the same size structure? Exposure to wastewater changes nearly every design decision, from the coating on the interior walls to the shape of the channel at the base. At Foley Products, we manufacture precast sanitary manholes for municipal and private wastewater systems, and engineers who understand these differences early avoid many of the change orders that come from treating a sanitary structure like a general-purpose one.
How Sanitary Manholes Differ From Storm and Combined Systems
Storm manholes handle intermittent flow, mostly clean water, and short-duration surcharge during heavy rain events. Sanitary manholes handle continuous flow, wastewater with corrosive properties, and gases produced by decomposing organic material inside the pipe network. That constant exposure calls for different materials, coatings, and sealing approaches than a storm structure of the same diameter and depth would need.
Hydrogen sulfide gas is the primary concern in sanitary systems. As wastewater moves through the network, bacteria produce hydrogen sulfide, which converts to sulfuric acid when it contacts moisture at the crown of the pipe and inside the manhole. Over years of exposure, that acid attacks unprotected concrete, so sanitary manhole specifications frequently call for protective linings or coatings that storm structures rarely need.
Inflow and Infiltration Control in Sanitary Manhole Design
Inflow and infiltration, often shortened to I and I, describes clean water entering a sanitary system through manhole covers, cracked joints, or deteriorated seals. Utilities treat every gallon of I and I at the same cost as a gallon of actual wastewater, so they track it closely and specify manholes accordingly.
Watertight frame and cover assemblies, gasketed joints between precast sections, and boots at pipe penetrations all reduce infiltration into a sanitary system. We manufacture precast sanitary manholes with joint sealing systems rated for the hydrostatic pressure a given site presents, and the specification should confirm which sealing method the receiving utility requires rather than defaulting to a general precast joint. Some utilities specify a particular gasket profile or sealant chemistry based on past performance in their system, and confirming that preference before manufacturing avoids rejection at inspection.
Bench and Channel Configurations for Flow Efficiency
The channel at the base of a sanitary manhole guides flow from the inlet pipe to the outlet pipe with as little turbulence and stagnation as possible. A poorly shaped channel creates dead zones where solids settle, and settled solids in a sanitary system contribute to odor, corrosion, and eventual blockages that require a maintenance crew to clear.
Bench slopes above the channel need to shed any splashed or standing wastewater back into the flow path rather than allowing it to pool. This detail is easy to overlook in a specification focused mainly on pipe sizing and structural loading. Still, it directly affects how much maintenance a structure requires over its service life. Drop connections, used when an inlet pipe enters significantly higher than the outlet invert, need their own design consideration since a poorly designed drop can create turbulence and accelerate corrosion at the point of impact.
Outside drop connections, where the drop pipe runs down the exterior of the manhole rather than inside the structure, are common in sanitary applications specifically because they keep the turbulent impact point away from the interior walls a maintenance crew has to enter. Specifying an outside drop instead of an inside drop on a manhole with a significant elevation change affects both corrosion exposure and long-term maintenance access, and confirming that choice against the receiving utility’s standard details before the structure is manufactured avoids a costly rework.
Coatings and Corrosion Protection for Wastewater Applications
Interior coatings protect concrete from the sulfuric acid produced by hydrogen sulfide exposure. Options range from epoxy coatings to polyurethane linings to calcium aluminate cement mixes designed to resist acid attack at the molecular level rather than relying solely on a surface barrier. The right choice depends on the system’s specific conditions, including flow velocity, retention time, and the hydrogen sulfide concentration the utility has measured in similar structures.
We work with engineers to specify coating systems matched to documented conditions in the receiving system rather than applying a single coating standard across every sanitary structure regardless of exposure. A manhole in a force main discharge point, where hydrogen sulfide concentrations tend to run higher, needs more aggressive protection than one further downstream in a gravity system with better ventilation.
Coating thickness and application method matter as much as the material choice itself. A calcium aluminate liner applied at insufficient thickness will not perform to its rated resistance, and field-applied coatings depend heavily on surface preparation and cure conditions that a plant environment can control more consistently than a job site can. Specifying the coating material along with the required thickness and application standard, rather than the material alone, gives both the manufacturer and the inspecting engineer a clear benchmark to verify against.
What ASTM and Local Codes Require for Sanitary Manholes
ASTM C-478 governs precast reinforced concrete manhole sections generally, and most state and local codes reference this standard as a baseline for sanitary manhole construction. Many municipalities also add supplemental requirements specific to sanitary applications on top of that baseline, covering everything from minimum coating thickness to acceptable joint-sealing methods to bench and channel geometry.
Foley Products operates 18 manufacturing facilities across nine states, and local sanitary sewer standards vary enough between jurisdictions that a specification written for one municipality often needs adjustment to apply cleanly to another. Confirming the governing local code, alongside the applicable ASTM standard, before finalizing a specification prevents a structure that meets general precast standards but falls short of a specific municipality’s sanitary sewer requirements.
Specifying for Decades of Reliable Service
Sanitary manholes operate in a harsher environment than most underground structures, and specifications written without that environment in mind often show problems well before their expected service life ends. Coating selection, joint sealing, channel design, and applicable local code must reflect the actual conditions of the system the structure will serve, not a generic template borrowed from a storm sewer project.
If you are specifying sanitary manholes for a municipal or private wastewater project, contact Foley Products directly. We can help confirm coating requirements, joint sealing methods, and local code compliance before you finalize your specification.
A sanitary manhole specification succeeds when it accounts for the chemistry and biology at work inside a wastewater system, not just the structural loads above ground. Precast sanitary manholes built with the right coatings, sealing systems, and channel geometry resist corrosion and infiltration for decades, while structures specified without those considerations often need premature rehabilitation.
Frequently Asked Questions
What’s the typical design life of a precast sanitary manhole? A properly specified and coated precast sanitary manhole can provide fifty years or more of service. However, the actual lifespan depends heavily on coating selection, hydrogen sulfide concentration, and how well infiltration is controlled at the joints.
Do sanitary manholes require special linings? Most do, particularly at force main discharge points and other locations with elevated hydrogen sulfide exposure. The specific lining or coating depends on documented system conditions, and a utility’s engineering standards typically dictate which options are acceptable.
How do engineers determine manhole spacing along a sanitary line? Local code requirements typically govern spacing, often based on pipe diameter and the practical limits of maintenance equipment used to clean and inspect the line. Changes in direction, slope, or pipe size typically require a manhole regardless of the standard spacing interval.
