Special Considerations: Hydraulics, Wastes and Sulfide

Sewers and culverts are designed to carry the design flow and to provide sufficient strength to support imposed loads at an economical cost. Hydraulic design requires determining the system type, determining the design flow, selecting the pipe size and determining the flow velocity. Design for safe supporting strength requires determining earth and live load, selecting a bedding, determining the load factor, applying a factor of safety, and selecting a pipe strength. This page covers the hydraulic and durability half; the structural half is at load carrying capacity, cracking and jacking.

Hydraulic coefficients of flow

Flow through sewers and culverts depends primarily on the size and slope of the line and the roughness of the pipe wall. The most important single factor in hydraulic design is wall roughness, which makes selection of the friction factor unusually consequential — an assumption made in a moment fixes the pipe size for the life of the asset.

Roughness retards flow and reduces velocity. Friction effects are commonly evaluated through the friction factor n in the Manning equation for open channel flow, with higher values indicating greater loss. A pipe whose interior surface produces minimum frictional resistance is therefore necessary for hydraulic efficiency.

Authoritative testing has established that the roughness coefficient of concrete pipe is equal to or better than that of other pipe materials hydraulically. Laboratory results indicate that the meaningful differences are between smooth-wall and rough-wall pipe rather than between smooth-wall materials: rough-wall or corrugated pipe carries n values roughly 2.5 to 3 times those of smooth-wall pipe. Test programmes have established values for concrete pipe from 0.009 to 0.011.

Recommended design values

Design practice does not use the laboratory value. It uses a value deliberately higher, to account for what happens to a real pipe in service:

• Sanitary sewers: n = 0.012 to 0.013. Allows for slime or grease build-up on the wall and for minor head losses at fittings and manholes.
• Storm sewers: n = 0.010 to 0.012. Allows for foreign debris and minor obstructions.
• Culverts: n = 0.010 to 0.012. Provides a reasonable margin of safety.

Using the laboratory figure in design is a genuine error rather than an optimistic choice: it produces a pipe that is correctly sized for a wall condition the line will not have after its first year. Open channel and culvert hydraulics guidance is published by the Federal Highway Administration, and the design data sheet series listed under design aids covers hydraulic capacity, equivalent flow capacity and partial flow conditions for each shape.

Industrial wastes

Certain liquid wastes from industrial processes have objectionable effects on the materials used for sewers, pumping stations, treatment plant equipment and structures, and on the treatment processes themselves. Such wastes should not enter the sewer except after pretreatment sufficient to prevent those conditions, and the practice of enforcing regulations against discharge of improper wastes is well established. Pretreatment requirements in Illinois are administered through the Illinois EPA.

As regards the pipe specifically, the picture is narrower than is often assumed. Generally the only waste that directly damages concrete pipe is acid. A common requirement in pollution control regulation is that the pH of a waste discharged to a sewer shall not be lower than 5.5; authorities state that concrete pipe can carry liquid with a pH as low as 4 without harm. Highly alkaline sewage has no adverse effect on concrete pipe at all.

Acid conditions arise where industries use pickling or other acids in plant processes. Such dischargers should be required to neutralise their acid wastes, or a protective lining should be provided on the carrier pipe as recommended by the manufacturer; epoxy-coated concrete pipe is available in most areas. In a well-designed sanitary system carrying industrial waste alongside domestic flow, dilution is generally sufficient to raise the pH of the mixture above any level that would affect the pipe.

Hydrogen sulfide in sewers

Much has been written about hydrogen sulfide in sewers, and the attention is warranted: it is highly objectionable in odour, it is toxic, it interferes with treatment plant operation, and when oxidised it corrodes several of the materials used in sewer construction.

Two qualifications belong alongside that. Hydrogen sulfide in amounts sufficient to cause damage is not common in sewers, and where it does occur it is generally present only part of the time. Its formation requires specific conditions — those which permit sewage to become septic, principally long detention time, warm temperature, low velocity and an available sulfate source.

The mechanism is worth stating because it explains where the damage appears. Sulfide is generated in the slime layer below the water line under anaerobic conditions, then released into the sewer atmosphere. It is above the water line, on the crown and the upper walls, that aerobic bacteria oxidise it to sulfuric acid, and it is there that concrete is attacked. A sewer suffering sulfide corrosion therefore shows a sound invert and a deteriorating crown — the reverse of what an inspector expecting abrasion damage would look for.

Because the conditions that generate sulfide can be predicted at design stage, the correct response is to predict them and, where the potential is significant, to specify a lining or coating from the outset. Systems available and their limitations are covered under protective lining products.