Sewage that has run out of dissolved oxygen and nitrate turns septic: sulphate-reducing bacteria in the slime layer convert sulphate to sulphide, and the sulphide leaves the water as hydrogen sulphide wherever there is turbulence. Consequences are odour complaints at the nearest manhole and, over years, the destruction of concrete sewers and pumping stations by the sulphuric acid produced on their crowns. This guide gives the mechanism, a method for estimating how much sulphide a rising main will generate, the corrosion and odour criteria, and a critical comparison of the control options.

How sulphide forms

Sewage carries sulphate (typically 30 to 100 mg/L from the water supply and from detergents) and organic sulphur compounds. In the aerobic water of a well-ventilated gravity sewer, the bacteria in the wall slime use dissolved oxygen and the sulphate is untouched. When the oxygen is gone, and any nitrate with it, sulphate becomes the next available electron acceptor and sulphate-reducing bacteria in the anaerobic depth of the slime layer, and in any deposited sediment, reduce it to sulphide. Sulphide so produced is partitioned by pH:

H2S ↔ HS− + H+, pKa ≈ 7.0 at 20 °C.
At pH 7 about half the dissolved sulphide is molecular H2S and can be stripped to the atmosphere; at pH 8 about 9 percent; at pH 6 about 90 percent.

Three conditions therefore control the problem: anaerobic residence time (rising mains, full siphons, deep slow sewers), temperature (the formation rate roughly doubles for each 10 °C rise between 5 and 25 °C, so summer is the design case), and turbulence at the release point (a discharge manhole, backdrop or cascade) where the dissolved gas comes out of solution.

Predicting sulphide build-up in a rising main

In a full pipe the sulphide generated is proportional to the slime area per unit volume, the organic substrate available and the temperature-corrected rate. Following Pomeroy and Parkhurst, the widely used form is:

ΔS = M × EBOD × (P/A) × t
ΔS = increase in total dissolved sulphide (mg/L); EBOD = effective BOD = BOD5 × 1.07(T − 20) (mg/L); P/A = wetted perimeter over cross-sectional area = 4/D for a full pipe (m−1); t = residence time (h); M = sulphide flux coefficient, typically 0.3 to 1.0 × 10−3 m/h for rising mains, with 0.5 × 10−3 a reasonable design value.

Worked example. A 300 mm rising main carries sewage with BOD5 of 250 mg/L at 15 °C in summer, and the pumping regime gives an average residence time of 2 hours. EBOD = 250 × 1.07−5 = 250 × 0.713 = 178 mg/L. P/A = 4/0.3 = 13.3 m−1. ΔS = 0.5 × 10−3 × 178 × 13.3 × 2 = 2.4 mg/L of dissolved sulphide arriving at the discharge manhole. At pH 7 half of that is H2S; the headspace of a turbulent discharge chamber can reach tens of ppm by volume, enough for odour complaints at the surface and for aggressive crown corrosion downstream. Increase the residence time to 6 hours, common at night or in a main sized for future flow, and the sulphide triples. Smaller pipes are worse per unit volume because P/A rises as D falls: a 150 mm main generates twice the sulphide of a 300 mm main for the same residence time.

This equation shows the design levers directly: residence time (pump frequency, main sizing, avoiding over-sized mains), temperature (nothing to be done), substrate (little to be done) and pipe diameter. For gravity sewers a self-cleansing velocity of about 0.75 m/s at average flow keeps the water aerobic and prevents the sediment deposits in which sulphide is generated even when the flow is not anaerobic.

Corrosion: how a concrete sewer is destroyed

Dissolved sulphide does no harm to the pipe. The damage starts when H2S escapes to the sewer atmosphere, dissolves into the condensation film on the crown and walls above the water line, and is oxidised there by sulphur-oxidising bacteria (Thiobacillus species) to sulphuric acid. That acid reacts with the cement matrix to form gypsum and ettringite, which have no strength and spall away, exposing fresh cement. Loss rates of several millimetres a year are recorded in warm climates, and complete collapse of concrete and asbestos-cement sewers has occurred within six to eight years; in the UK the same process runs more slowly but is the leading cause of concrete sewer and pumping station deterioration downstream of rising mains.

The pattern is diagnostic: corrosion at the crown and above the waterline, worst just downstream of a rising main discharge, at drops and in poorly ventilated sections, with the invert intact. Metalwork fares no better: H2S blackens copper and brass and attacks electrical contacts, so the control panels in a wet-well building are an early casualty. Protective linings (HDPE, calcium aluminate mortar, epoxy) stop the attack but not the gas; the engineering answer is to stop the sulphide reaching the atmosphere.

Odour: criteria and assessment

Sewage odour is a mixture: hydrogen sulphide and organic sulphides (odour thresholds of about 1 to 4 parts per billion) give the acidic, rotten-egg character; ammonia and amines are the alkaline, more persistent component. Odour concentration is measured by dynamic olfactometry in European odour units per cubic metre (ouE/m³), the number of dilutions to the detection threshold of a panel. The UK has no statutory numerical odour standard; the Defra code of practice for sewage treatment works sets the test as "no reasonable cause for annoyance" under statutory nuisance law, and the Environment Agency's horizontal guidance for permitted sites uses benchmark 98th-percentile hourly-mean concentrations at receptors of 1.5 ouE/m³ for the most offensive odours (which includes sewage sludge and septic sewage), 3 for moderately offensive and 6 for less offensive odours, assessed by dispersion modelling.

For a discharge manhole or pumping station the practical assessment is a sulphide profile along the main (sampled at the discharge and preserved immediately with zinc acetate), headspace H2S logging at the release point, and a complaint log mapped against wind and temperature. A modelled receptor concentration above the benchmark, or a complaint history, triggers control.

Control options compared

MethodMechanismStrengthsLimits
Reduce residence timePump more often, smaller or twin mains, avoid oversizingNo chemicals, permanentConstrained by existing assets and pump cycling
Nitrate (calcium or sodium nitrate)Supplies an alternative electron acceptor so sulphate is not used; also oxidises sulphide already formedReliable, widely used, no odour of its ownDose must match oxygen demand along the whole main and vary with flow and temperature; overdosing wastes chemical and adds nitrate to the works
Oxygen or air injectionKeeps the sewage aerobicCheap gas, no residual chemicalLimited solubility; gas pockets in rising mains; needs pressure to dissolve enough
Iron salts (ferrous or ferric chloride or sulphate)Precipitate sulphide as iron sulphideEffective on existing sulphide; the iron also helps phosphorus removal at the worksDoes not stop formation; corrosive handling; black solids; dose rises with sulphide load
Magnesium hydroxide or caustic (pH elevation)Raises pH so sulphide stays ionised as HS− and is not stripped; slurry also inhibits bacteriaMagnesium hydroxide is safe to handle and self-buffering around pH 9Caustic to pH 9 at all times is expensive and hard to control; raises alkaline odours; works must accept the pH
Biocides (hypochlorite, chlorinated organics)Kill the slime layerFast shock treatmentContinuous dosing needed, high doses, harmful to downstream biological treatment and digestion
Ventilation and odour treatment at release pointsExtract and treat the sewer air (carbon, biofilter, chemical scrubber)Treats what escapes regardless of causeDoes nothing for corrosion upstream of the extraction point; capital and maintenance

For a long rising main the usual answer is nitrate dosing at the pumping station, with the dose set from the sulphide profile and trimmed by a sulphide or redox measurement at the discharge, backed by ventilation and carbon treatment at the discharge manhole. Dosing is a chemical-handling duty with the same requirements as any other chemical dosing installation: bunded storage, calibrated pumps, flow-paced control and a record the regulator can inspect. Where septic tankered waste is discharged into the network the problem arrives ready-made; its management is covered in the flows and loads guide, and the treatment-works end of the odour question in the odour control guide.

How to assess and control septicity on a rising main

  1. Establish residence time. From main volume and pumped flow pattern, including night-time and dry-weather minima; the maximum residence time is the design case.
  2. Estimate sulphide generation. Apply the flux equation with summer temperature, measured BOD and the pipe diameter; compare with sampled sulphide at the discharge, preserved on sampling.
  3. Assess the release point. Log headspace H2S at the discharge manhole, inspect the crown and metalwork for corrosion, and review complaints against weather.
  4. Fix the hydraulics first. Shorten residence time by pump control or main changes; remove sediment traps and unnecessary drops; ventilate release points.
  5. Select and size the chemical control. Nitrate for prevention, iron or pH elevation for existing sulphide; size from the oxygen demand or sulphide load with flow pacing and seasonal adjustment.
  6. Monitor and adjust. Sulphide or redox at the discharge, headspace H2S trend, chemical use per cubic metre pumped; retune each season.

Frequently asked questions

Why do rising mains cause more sulphide than gravity sewers?

A rising main runs full, so there is no air surface to re-oxygenate the sewage and the entire wall is covered by an anaerobic slime layer; residence time between pump runs can be hours. Sulphide formed there stays dissolved under pressure and is released all at once at the turbulent discharge manhole, which is where odour and corrosion appear.

How does hydrogen sulphide corrode concrete?

Not directly. H2S escaping into the sewer atmosphere dissolves in the moisture film on the crown, where sulphur-oxidising bacteria convert it to sulphuric acid. That acid attacks the cement matrix, forming weak gypsum and ettringite that spall off. Damage is therefore above the waterline, worst just downstream of rising mains and at turbulent drops, with the invert intact.

What is the best chemical for controlling septicity?

For preventing sulphide formation in a rising main, nitrate dosing at the pumping station is the most widely used and reliable, provided the dose covers the oxygen demand over the whole residence time. For sulphide that has already formed, iron salts precipitate it and magnesium hydroxide keeps it ionised so it is not released. Biocides are a last resort because of their effect on downstream treatment.

Is there a UK limit for sewage odour?

No statutory numerical limit. Statutory nuisance is the legal test, with no reasonable cause for annoyance as the aim of the Defra code of practice. The Environment Agency guidance for permitted sites uses benchmark 98th-percentile hourly concentrations at receptors of 1.5 odour units per cubic metre for the most offensive odours, including septic sewage and sludge, rising to 3 and 6 for less offensive odours.

Sources & further reading