Old, smelly and dangerous
Updated: Jun 4, 2020
Sewage is best moved on for a variety of reasons.
Some of those may appear obvious.
One that is not so obvious is that it can become dangerous.
The below information from Wessex Water is a succinct account of what can happen.
Foul sewage is waste from domestic kitchens and bathrooms, or commercial businesses. A common problem with foul pumping stations is a combination of low flows and long retention times. This results in bacteria multiplying in the anaerobic conditions. This is called ‘septicity’ and can occur in wet wells or rising mains. Consequent formation of hydrogen sulphide gas (H2S) can cause:
· A nuisance with its characteristic ‘bad egg’ smell, which leads to customer complaints.
· Lethal gas hazard during man entry into chambers.
· Reaction with moisture to form sulphuric acid. This has a corrosive effect on pipeline linings, concrete chambers and electrical fittings.
The secret is to keep things flowing.
Why are low flows a problem?
Pump stations as over against gravity sewers always have some sewage inside them and inside the pipework leading from them. If the flow coming into them is infrequent then some sewage will be sitting there for some time. If the flow is low then it may take a while for enough sewage to get into the system to keep flushing the pipe through.
Where might this be the case?
One example of this is a community hall where people gather maybe once a week in small numbers or a sports club that only operates at the home ground once a fortnight during a season. During opening hours the toilet and/or washing facilities may or may not be heavily used and once closed which will be a majority of the time sewage will be siting in the pipe and the chambers.
What is a long retention time?
Retention time is the length of time that the sewage is left sitting for. In the circumstance of the sports club this could be nearly two weeks. It is normally measured in hours and it is suggested that where retention times exceed 6 hours septicity is likely to be a problem.
What can be done?
In a lot of these cases we suggest before locking up the system is washed through with clean water where practical. Paradigm design engineers can assist with this calculation where required.
What if this is not practical or is too costly?
On new systems the next option is to look at the design to see whether the rising man diameter and chamber size could be reduced. Where this is not practical and the system is to be adopted the water authority will ask for septicity controls measures normally in the form of a chemical dosing system. Typically, this is supplied with a pumping unit and complete with a liquid storage tank and a bunded tank and will be automatically set up to dose at the required intervals. It is important that this is linked with any telemetry system to advise operators when levels are getting low to re-order refills. Where the system is not to be adopted this may be an expensive option to purchase and run and we are happy to have the discussion about the next steps.
Why does this all occur?
When sewage is not in movement for an extended period it can start to break down which releases dangerous chemicals. It is possible for a mixture of these to start attacking the integrity of the internals of the pump station leaving for expensive repairs to be completed by competent persons in a highly dangerous atmosphere.
Other ways to reduce septicity and minimise retention time of sewage in transport under anaerobic conditions:
1. Minimise the length of the rising main
2. Ensure that the slope of gravity sections prevent sedimentation
3. Minimise intermediate storage
4. Prevent seawater intrusion
5. Avoid siphons
6. Avoid untreated putrescible and warm wastes from industrial sources
7. Improve ventilation
8. Air stripping at the inlet works and treatment of the stripped air.