A Basic Primer on Sewer Systems
Most of us do not think much about what happens after we flush the toilet. But there is a world of infrastructure out there that may come with a $20,000.00 price tag per Island household in the near future.
To be well informed about the issue, we thought we would begin with a brief primer on central sewer. These are the three types of central sewer systems, all of which are used to transport raw sewage through pipelines to a treatment facility.
Gravity sewers use large pipes trenched in a slow downgrade to move sewage. They are used in extreme high-density areas, such as you would find in major cities and municipalities – not applicable on the Island.
A Vacuum Sewer system maintains a partial vacuum inside the pipe network and vacuum station. A single central vacuum station (also called a lift station) can collect the wastewater of several thousand individual homes, depending on terrain and the local situation. Vacuum systems can be found in flat areas which have a high water table. Pipework is laid relatively flat in the ground and the whole intention of this type of system is to ensure that construction remains as shallow as possible. This reduces costs, time and environmental impacts.
Low Pressure Sewer Systems (Grinder Pumps) use a small pump station located at each house to move wastewater through the transmission system. Low-pressure systems (grinder pumps) can often be used when connecting outlying homes or developments to municipal central sewer. However, there are downside considerations with grinder pumps:
- 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. 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 the County.
The following is a table of comparison between low-pressure systems (grinder pumps) and vacuum system, Please note that the information in the cost section is a generalized, nationwide cost estimate and can be expected to be out-of-date and/or not applicable to our Island.
Sewer System Comparisons: Vacuum or Low Pressure Pumps
POWER REQUIREMENTS
VACUUM SYSTEMPower is required only at the Pump Station, so a back-up generator can be supplied if required. A single pump station is usually all that is required to cover a large area, thus reducing overall power costs versus gravity. Pumps only run on average 3 hours per day so power use is lower than alternative systems. |
LOW PRESSURE SYSTEMGrinder pumps require power at each unit. The panel is usually installed on the side of the house and power paid by the homeowner. Existing houses may need to upgrade electrical mains and power board. Some pumps require constant power. Systems that require transfer pump stations will require high total power use. |
CONNECTIONS
VACUUM SYSTEMUsually 4-6 homes are connected to a single collection pit, though larger flows from a gravity area can be accommodated into a multiple collection pit. Hotels, schools and high flow areas can be serviced. |
LOW PRESSURE SYSTEMOne pump is required per house. If the house is large or commercial flows anticipated then a larger capacity pump may be required. Camping grounds and high flow situations are not recommended. |
EXFILTRATION
VACUUM SYSTEMSince vacuum is maintained within the mains at all times there is no chance of exfiltration of sewage. Operators are immediately alerted of a leak in the main due to longer vacuum pump run times and the leak can quickly be found and repaired. |
LOW PRESSURE SYSTEMAs all pipework is under pressure then a break in the pipe will lead to large exfiltration. Not suitable in environmentally sensitive areas. |
INFILTRATION
VACUUM SYSTEMAs it is a vacuum tight system, infiltration cannot occur. |
LOW PRESSURE SYSTEMAs it is a pressurized system infiltration cannot occur. |
MAINTENANCE
VACUUM SYSTEMIf service is required a single operator takes on average 10-15 minutes to complete any service required. No special equipment is required. MTBSC = 20 Years. Parts costing approx. $200 required for the interface valve each 10 years. As all services are on public property accessibility is not a problem. 10 Year warranty is supplied |
LOW PRESSURE SYSTEMMost callouts require the pump to be lifted out. An electrician is needed to be part of the service team. Access is difficult as all equipment is on the homeowner’s property. MTBSC =8 years One Year warranty only supplied for certain components. Sand and Grit will cause faster wear on the grinder cutters. If it is in a high flow situation some parts may require annual change. |
COST
VACUUM SYSTEM100 lots = $10,000 |
LOW PRESSURE SYSTEM$8,500-$12,000 per lot though in some country areas as high as $18,000 per lot. |
