Gravity sewers have been used for hundreds of years, but they come at a much higher cost than many options reports accurately record. When selecting what type of sewerage system to use, take into account the risks associated with the solution. What happens if a sewer overflows? What happens if a storm or large wet weather event occurs. What will happen if a pipe breaks. What will be the cost and what will be the risk? A vacuum sewer could be far cheaper than you realise.
When you reflect on the damage and the cost of exfiltration from sewer networks the statement that we often hear ‘gravity sewers require no maintenance – we just put them in and leave them be” is wrong and does not reflect the true risks of not only gravity systems, but also low pressure systems as well as old septic tanks and combined sewers.
Sewage leaks can occur for a number of reasons. the most common is leaking sewer pipes (this may be caused by tree roots or ground movement). A break in a gravity line can remain undetected for months or even years. In a Low Pressure Pump system a break will lead to a surcharging sewer which will be difficult to repair and costly.
With Low pressure systems a pipe break is easier to detect as some local TV news anchor is excitedly showing the 50 foot geyser in the middle of main street.
Overflows can also be caused by the system being inundated with stormwater which overwhelms the entire system allowing polluted stormwater to enter waterways.
A vacuum sewer piping network is leak tight, as it operates under a negative pressure any break cannot allow exfiltration to occur and as the pipe is less than 1/3 full at any time there is no surcharge.
What are the main problems if exfiltration occurs?
In the USA the Centres for Disease Control and Prevention (CDC) and the US EPA regularly report on annual deaths due to sanitary sewer overflows. Sickness is widespread with the most common symptoms being diarrhea and nausea. This can be caused not only by drinking the water but also by swimming and eating shellfish and fish.
Of greatest concern is infectious disease, such as the contamination of Wallis Lake oysters in Australia with the sewage-derived virus hepatitis.
In Baradine a small town in Australia The Department of Health reported a 30% reduction in student sick days at the local school once a new vacuum system was installed. In far north tropical Queensland the introduction of a vacuum system led to a reduction is diseases caused by mosquitoes.
In the nineties nearly 20% of Florida’s manatee population died due to an algae bloom which had intensified due to sewer overflows.
On average 140 beaches in the US are closed or have warnings due to sewage contamination. KawaKawa Bay in New Zealand which had been a source of shellfish and a place to swim for the indigenous Maori population for hundreds of years was completely closed for ten years due to sewer overflows from a growing seaside community. It was only after a vacuum system was installed that the bay was reopened.
Sewage contamination readily degrades in the environment, but in water it can soak up oxygen from the waterway. Low oxygen levels can kill large numbers of fish and other aquatic life. Sewage also has toxic levels of ammonia that can poison aquatic life, along with many other contaminants.
Overflows contribute nutrients and sediment that encourage the vigorous growth of invasive weeds, such as willows, blackberries and privet, which often choke native species along stream channels and banks.
Aquaculture and Tourism Impact
Owners of Marina’s know the impact of attracting houseboats and tourists if sewage is contaminating the marina area, with bad smells putting off people going to local restaurants.
Dal Lake, one of the most scenic tourist spots in Kashmir has suffered terribly from sewage overflows from nearby towns but even worse from raw sewage entering the lake from houseboats on the lake.
Tourists going to Brazil for the Olympics and the World Cup were told not to go near the beaches. Dr Daniel Becker, founder of the nonprofit Center for Health Promotion in Rio, said: ‘In Rio, if you’re going to the beach, you’re going to sewage.’
“A sewer line clogged with grease caused raw sewage to spill into the Five Mile River , forcing the closure of shellfish beds from Darien’s Long Neck Point to Wilson Point in Norwalk.” This is a common story, found around the world where sewage overflows can affect not only health but the livelihood of
Financial and Reputational
In many countries fines are being issued to water authorities if sewage overflows occur. The Environmental Protection Agencies must report any overflows and state and federal reporting is mandatory. If there is a death related to a sewage overflow that could of been prevented then lawsuits and possibly prison could affect senior people within the wastewater industry.
Newport Bay in California had to close its beach to swimmers and divers on one of its busiest weekends due to a large sewage spill that was caused by a sewage pump stopping, flooding the streets and beach. Not only was the beach closed for 4 days, ruining many tourists holidays, it also financially hurt local small businesses including restaurants who are talking about legal action against the Costa Mesa Sanitary District. The District has now been fined $503,000 by the regulators of The Santa Ana Regional Water Quality Control Board. Sadly it didn’t end there with a smaller spill caused by tree roots blocking a sewer main causing beaches to close for three days the following year.
There are always lots of big fish down at the lake because…………..
The first problem, is how do you know that you have a leak or overflow.Usually it is because sewage has been spotted at the local playground/beach/street. Or because you know from flow meters at the Treatment Plant that not all the flow is reaching it. In New Delhi it is estimated that only 60% of all sewage collected is actually reaching the treatment plant.
Even with new technologies it is difficult to find leaks when the pipes are buried 10 metres (40 feet) underground. It is also very expensive to fix that leak. Not only is there potentially a disruption to service, a closing of roads the restoration of the surface, you need to find an allocation in the budget to make those repairs.
Why doesn’t all this happen with a vacuum sewer?
Well admittedly, homeowners can still create overflows at the home and badly maintained systems will always have problems. Modern wireless and cable monitoring systems reduce this risk considerably and is a game changer.
Firstly though, all vacuum mains are fairly small diameter, 125, 160, 200 dia (6,8,10 inches) and laid at a very shallow depth (Under 1.5m or 6 feet) So if there is ever a break it can be reached quickly and repaired without loss of service or expensive restoration. All pipework has to be pressure rated so it is a lot thicker than traditional sewer pipe.
Sewage is transported quickly in the pipe by air. The pipe is full mainly of air and not sewage. Large sections of the pipe are empty with sewage only collecting at lifts in the system. All pipework is under a negative pressure created at the pump station. If a pipe breaks, it sucks in and does not let any water leak out.
Once there is a leak or break in a pipe, the vacuum pump at the vacuum pump station must run longer. This creates an alarm which is transmitted to the operator. He is made aware that a leak is occurring in the system, this may be at the vacuum pump station, at a valve pit or in the pipework. A reticulation monitoring system will automatically notify the operator if there is something blocked in a valve or there is a break in the line. When pressure is monitored at each pit it is easy to detect in which section between two pits a break has occurred.
If a break has occurred, it would usually be due to an outside contractor breaking the pipe, tree roots rarely get into vacuum mains as they are pressure pipes. As a break is at minimum depth it is easy to locate and repair and no sewage would have leaked out.
If a blockage has occurred and sewage is backing up in the pit it cannot overflow through the manhole cover as there is a partition between the sump and cover preventing sewage from flooding at this point. If it starts to back up towards the house a high level alarm in the pit sends an urgent alarm to an operator who attends to the problem. If the problem is hard to solve or an experienced operator is not available a suction truck can be used at this point.
At the Eco Resort near the Barrier Reef in Australia a vacuum system was chosen to ensure that no sewage could affect the local ecology nor leak into the swimming lagoons which would have closed the resort.
As any leak in the pipework, must be repaired immediately, no stormwater can enter the system during wet weather events. This is common in gravity systems where breaks may go unnoticed. This means that the vacuum system can never be inundated with stormwater and wet weather infiltration is not included in design parameters.
Due to the velocity of water in the pipes there is not build up of fats or grease and wet wipes or other objects do not cause blockages at the vacuum valve nor in the pit. See Video for information.
Vacuum Sewers are considered to be the Low Cost of Sewage collection, both in capital costs and also in maintenance costs. Risk is a very important consideration.