What is the right type of sewer system for your project?
Although gravity sewers are the preferred choice for many utilities, infiltration and inflow (I&I) has become one of the most significant factors that are forcing utilities to look for choices that will reduce water getting to the treatment plant affecting issues surrounding discharge options.
It is important to rationally determine what the best sewer system for your project would be. In many cases it may be a combination of some or all, that may the best way forward.
Municipal sewerage systems (either in new developments or as system replacements) will more than likely be a choice between:
Low Pressure Pumping Systems
Vacuum Sewer Systems
An analysis of a project will usually include the capital cost of the system and the whole of life costs which will include energy costs and operational costs. Quite often analysts are also doing carbon footprint comparisons as well.
Septic Tanks or Septic Tank Effluent Drainage (STED) or Septic Tank Effluent Pumping (STEP)
Although cheap to install most municipalities have banned the use of septic tanks and the use of drainage or effluent schemes is done only as a short term measure as replacement is usually required within a 10 year period. The main issue is the reliance on the homeowner. This can result in overflows to wetlands, water supplies or rivers creating health risks and environmental damage. (Read about Septic Tank Replacement Schemes)
Grinder Pump or Low Pressure Sewer Systems
Grinder Pumps can be an ideal solution for small communities of under 100 houses especially when located in hilly areas where large housing blocks are spaced apart. Pumps can quite often be used in combination with vacuum or gravity schemes when wanting to attach small outlying communities or houses to a municipal scheme. The use of trenchless pipe installation can be attractive in certain areas as this is not possible with vacuum or in most gravity installations.
There are a few reasons why grinder pump systems have been banned from use in communities.
- Power Use. In a power outage there is no back up power supply available leaving the community without service for as long as it takes to rectify power. Even when power comes back on sudden surges from a whole town coming back online may impact the power system again. Both gravity pump stations and vacuum pump stations can be provided with a back up generator ensuring no loss of service.
- Septicity. In seasonal communities or in staged private developments sewage can sit in a force main for a considerable time before getting to the treatment plant.
- Access. Generally grinder pumps are situated on private property. For any repairs or replacements utility operators are required to go onto homeowners property to do any maintenance which may result in sewage spilling over gardens or lawns. Access may also prove problematic if there are dogs or after dark. All gravity and vacuum sewer infrastructure is on public land so no access is required. Pumps located within the property can be costly if easements are required to be paid for by council.
As pumps are generally located on the home owners property and operational and power costs often need to be met by the homeowner there is often a negative reaction by communities when other alternatives such as vacuum are available.
Usually communities have been left unsewered because the area is too difficult or expensive to install a gravity sewer. Gravity sewers have been used for hundreds of years but moves to improve health and safety of operators, reduce power and water use, reduce I&I and reduce environmental damage has led to a movement away from their use.
As the name suggests large diameter pipes rely on a movement of sewage via gravity down a pipe until it is too deep then a pump will bring it back to the surface where it begins the process again. There are a number of problems with this. As old systems have broken pipes, sewage contaminates ground water and fishing areas causing health problems and stormwater enters the sewer system churning energy with both pumping and treatment.
One of the most complex problems with gravity sewers is in its construction. Many systems require deep excavation, going down as deep as 20 meters (70 feet) or multiple pump stations which require land and power. In many towns the space is simply not available. Water, stormwater, power and communications cables also need room under the street. Construction time can be costly and time consuming, not good for developers nor local residents. Imagine 100 foot a day of excavation through coffee rock or acid-sulfate soils.
It is very common to read in the press of “fat icebergs” building up in the sewers or of rat infestations. Do operators really need to put up with this in the 21st century? Do they need to risk their lives in gas explosions from gas build-ups’ in the sewer?
There has certainly been a perception that vacuum systems are new and are difficult to maintain. Standards in many countries, including Europe, have required monitoring of the full vacuum network including valves,pits, pipework and pump stations, for a number of years now, this has been a gamechanger for vacuum sewers. Not only for troubleshooting but more importantly for detecting I&I which is significant for many utilities now having to grapple with the loss of ocean discharge as an option. (System Monitoring)
Vacuum sewers have also been around since the late sixties and seventies and the technology and design has also changed considerably. Many older systems used 2 inch valves and used siphon style design with their pipework. This led to increased blockages and flooded lines, very different from the systems that are available today.
Typically vacuum systems are found in area’s where it has been too hard to install a gravity sewer. Have a look at our page Types of Projects for the variety of vacuum projects which are being done. Today you can also see them being used to supplement overburdened gravity sewers in fast growing towns and cities, or in area’s which succumb regularly to earthquakes, floods and hurricanes. Replacement of old gravity sewers that are succumbing to climate change and rising water levels.
Vacuum Systems are often found in flat areas which either have a high water table or difficult excavation due to hard rock. But there are many other reasons for why a vacuum system may be the system of choice for your project. The average size project has 2,000 people but can be installed for 100 people or 100,000 people.
There are a variety of reasons why vacuum systems have been chosen on these projects, capital cost is a major one, as is ongoing power costs.
Advantages for the Contractor
- Pipes (vacuum mains) are laid in shallow open trenches at an average depth of 3-5 feet (1-2 metres)
- Small diameters pipes
- No manholes or lift stations required
- Rapid construction
- Design Flexibility
- Obstacles in the field can be avoided.
- Less dewatering (which can lead to lower costs for tankering out excess water)
- Significant cost savings
- Less danger to workers.
Advantages for Utility Owners and Developers
- Easy staging of infrastructure
- Versatility with design and installation
- Very fast installation time
- Leasing options can match project cash flows
- Environmental benefits
- No I&I means smaller discharge requirements
- Grey and black water separation possible
Advantages for the Operator
- Power only required at one central location
- Limited or no contact with sewage
- Monitoring showing pressure within the system as well as location of any breaks or blockages.
- No build up of H2S or other gases
- reduced operational costs
- No exfiltration of sewage
- Easy access to valves and pipework
- No confined spaces
- No fats and grease blockages
- No impact or blockages from flushable wet wipes or other items.
Advantages for the Environment
- No Exfiltration
- No Infiltration
- Low Energy Use
- No impact on the water table during construction
- Facilitative technology for water savings
- No odors
- Suitable for environmentally sensitive areas
- Ammonia and nitrogen reductions in collection and transport process
- Black and grey water separation possible.
- versatile installation of pipework allows for avoidance of sensitive area’s