Balancing environmental responsibility while expanding wastewater management infrastructure

By Mackenzie May

Traditionally, two methods of wastewater management were widely used, and they still are today. In developed areas, gravity fed sewer systems are used to remove wastewater from homes and buildings and deliver the fluid to collection points and treatment facilities.

A gravity sewer system consists of a network of large diameter pipes installed at a continuous downward grade, utilizing the forces of gravity to transport fluids, and lift stations to move fluid to a higher elevation when it is no longer reasonable to excavate deeper.

In areas where sewer infrastructure has not been widely built out, septic sewer systems are used to treat water and dissipate the clean fluid into the surrounding ground, called a leach field, while solids and sludge must be pumped out with a vacuum truck once every three to five years. While each of these technologies have their place, they also have their challenges in a variety of applications.

Pressure sewers offer an alternative sanitary sewer option because they provide an economic solution to challenging environmental conditions, where gravity sewers are impractical and where septic systems pose risks to the environment.

Pressure sewer systems utilize a network of grinder pumps to transport wastewater through small diameter pipes to a collection and treatment system. A grinder pump is a submersible pump designed to reduce the size of solids in wastewater to a slurry that can then be transported for treatment.

These grinder pump stations are located in the yard or basement of each home and wastewater flows into the basin from the building’s sewer line. The basin contains a grinder pump, level sensors, valves and discharge piping. Discharge lines tie into a central line, still small in diameter, that transports the fluid under pressure to either a collection point or a treatment facility. Pressure sewer systems can range from a handful of pumps and stations to tens of thousands of pumps and stations, making them very flexible. Environmental impact, accessibility continued overleaf… of infrastructure, cost and future development are all considerations that must be taken into account when planning new construction.

The installation of both gravity and septic sewer systems requires major excavation. With gravity fed systems, large diameter pipes must be accurately installed along a continuous downward grade to keep fluid moving at a high enough velocity that solids cannot settle in the line. When lines become too deep for reasonable excavation, lift stations are installed to transport fluid to a higher point of elevation.

Meanwhile, a septic tank installed underground has a capacity of at least 4,600 litres, with a leach field of at least 74.3 square metres.

Since pressure sewer systems are not limited by gravity, wastewater is pumped through small diameter pipes that follow the contour of the land. Piping is installed via directional boring in shallow trenches, located just below the frost line. This allows for minimal installation footprint and expedited environmental recovery.

With a pressure sewer system, wastewater can be transported several thousand metres and can be discharged at a point of higher elevation. As a result, the need for lift stations can be minimized or eliminated in almost every installation. Since directional boring eliminates the need for large trenches, it is possible to install pipe under existing infrastructure such as roads.

Internal layout of a typical residential pressure sewer system. These systems have an average maintenance interval of 10 years.

This simplifies installation or restoration efforts and costs compared to gravity systems, where existing infrastructure would need to be removed and replaced.

As a closed system, pressure sewers are also protected from risks of leaking, which can contaminate lakes, streams and oceans. A significant concern for failing septic install bases is the poten- tial for contaminating groundwater with bacteria such as E. Coli, which could be catastrophic for drinking water supplies or could pollute surrounding water in lakeside and ocean-front locations.

Greater Accessibility

For some, accessible wastewater management services mean physically being capable of receiving service. For others, accessible wastewater management services mean having services that a community can afford to maintain and operate. Pressure sewers can help to address both these accessibility concerns.

Since pressure sewer systems can travel long distances and operate in all terrain types, they provide the perfect solution for unique site challenges that builders face with traditional gravity systems. Normally hard-to-service locations include settings which are rocky, hilly, have high water tables, or have long stretches of flat terrain. These installation locations are typically unable to be served by gravity fed sewer systems, due to the network of large diameter sloping pipes that are required.

New developments are not the only place that deserve access to wastewater management infrastructure. There are many small rural communities across Canada that do not have the capital to invest in new or improved gravity sewer systems, which are expensive.

A typical lift station includes a wet well, screens or grinders for solids, pumps or compressors, valves, power supply systems, alarm and control systems, odour control and ventilation systems. The upfront cost to install a new lift station is typically at least $100,000. This does not account for additional maintenance and operational costs that will be incurred over the life of the system. Some small communities simply cannot afford this.

For many of these small rural communities, septic systems are prevalent. However, septic tanks often only have a life of about 30 years and replacement of a septic tank and leach field is also very costly.

Pressure sewer systems are a solution for septic abatement projects due to low costs, quick installations, minimal environmental disruption and restoration, and easy maintenance and operation. These systems have an average maintenance interval of 10 years.

Grinder pumps are typically designed to require less tools and equipment for troubleshooting and repair compared to the solids handling or chopper pumps found in lift stations. Compared to lift stations, pressure sewer systems do not require the same level of preventative maintenance and inspections.

Pressure sewer systems typically use one common pump design in the entire project, simplifying service and allowing “safety inventory” of repair pumps for instances of pump failure. This allows municipalities to save on both operational and maintenance costs.

The further benefit of pressure sewer systems is allowing developers to defer their upfront installation costs. Rather than installing a costly lift station well in advance of new construction, contractors can simply install the low-pressure sewer pipe up front, and defer the cost of the grinder pump station until the sale of the home. Theoretically, the sewage system can be commissioned the day the new owner takes possession of a home.

Finally, pressure sewer systems help with load management in treatment plants. Septic sewer systems must be pumped out. Trucks bring the waste to a treatment facility, creating an increased treatment load.

Furthermore, in-flow and infiltration are common issues in gravity sewer sys- tems because they are open to the environment. This drives up cost and inefficiency, as the treatment plant is taking on additional load by treating non-wastewater fluids.

Wastewater treatment plants for pressure sewer systems are less costly to build because the system is closed to infiltration and sizes of solids are minimized. Grinder pumps simplify the treatment process because the wastewater is already ground into a slurry, so solids are eliminated before reaching the plant.

From the homeowner perspective, a grinder station on their property includes storage capacity for one or two days in instances of power failure and will add only about two dollars per month to an electric bill.

Pressure Sewers Allow Future Growth And Development

Pressure sewers are a unique solution because they can be used in new applications and areas without existing infrastructure. They can also tie into existing sewer infrastructure. This helps to “futureproof” areas that may be inclined to experience continued population growth.

Rather than expanding a gravity sewer system and undertaking a large-scale project that may require significant road construction, the new development can utilize a pressure sewer network and tie that into the existing gravity sewer infrastructure.

The ability to continue to tie into existing systems also means that pressure sewer infrastructure can be built out in phases. Large development projects can take several years for complete construction. If using a gravity sewer system, the complete build-out of sewer infrastructure must be complete before home construction begins. With pressure sewers, large developments can be broken into many phases that ultimately tie together. This allows a developer to defer costs on longer-term build-out, until they have received revenue from earlier phases.

Mackenzie May is with Crane Pumps and Systems. Email: mmay@cranepumps.com