Where all wastewater servicing is provided on-site (within the property boundaries of the facility generating the wastewater flow), the collection system will consist of internal sewers carrying wastewater to the treatment and land-application system. For example, household drains from the dwelling to the septic tank, and then effluent lines from the tank to soakage trenches. Sometimes pumps or siphons are used to dose the trenches with the septic tank effluent.
Where wastewater treatment is off-site, the wastewater needs to be collected and transported to the treatment plant. This is done by a pipeline or network of pipelines (sewers) that collect wastewater flows from all dwellings in the community. Energy to transport the wastewater may be by gravity, pumping or a combination of pumping and gravity, depending on topography, layout and economics. For systems involving some on-site pre-treatment (such as a septic tank or grinder pump), reticulation is often done by pumped small-bore pressurised pipeline systems. This usually reduces the capital cost of the collection system, but will mean higher on-site costs. Other options include vacuum collection and transport systems.
Conventional sewerage (CS)
In the conventional system, household on-property sewer-lines (100 mm diameter) connect to street sewer-lines (minimum 150 mm diameter), which are reticulated in straight lines between manholes that provide access at all changes in direction. Manholes are used at all street reticulation connections to main collecting sewers, and again where trunk sewer connections are made. The minimum sewer sizes are based on design rules for use of traditional clay and concrete sewer pipes, and on the self-cleansing gradients necessary to scour out any sand and sediment entering the sewerage system.
The maximum distance between manholes historically has been determined by the need to mechanically clear obstructions using rods, which becomes difficult for distances over 90–100 metres. Manholes are a significant proportion (15–20%) of the total sewerage costs. They are also a point of weakness in conventional reticulation systems, as the manhole sewer connections often crack as the result of ground settlement and traffic impact, with groundwater infiltration then entering the sewer lines. Infiltration flows dilute the untreated wastewater flow, and result in diminished treatment process performance.
Modified conventional sewerage (MCS)
The availability of smooth-bore long-length 'plastic' pipe drainage lines has led to the development of MCS systems, which are particularly suitable for smaller communities converting from on-site to cluster or central sewerage and treatment. Manhole numbers can be reduced significantly through the use of rodding 'eyes' (inlets), and pipeline gradients and alignment can be varied to better fit the topography, thereby reducing construction costs. Minimum line diameters can be reduced from 150 to 100 mm depending on connection numbers, and self-cleansing gradients can be reduced due to the smoother pipe material, again producing construction economies. Infiltration opportunities are reduced, thus decreasing the wet-weather flow in the sewer and the hydraulic impact on the treatment plant processes. Flexibility in sewer line location in existing communities can be provided, such that the gravity sewer bypasses properties in low-lying areas, which are then connected to the sewer through a grinder pump unit and on-property rising main.
Effluent drainage servicing (EDS)
EDS was introduced into New Zealand in the late 1970s as a local version of the Australian CED (common effluent drainage) and the US STEP (septic tank effluent pumping) systems for reticulating septic tank effluent from each property, and conveying it off-site for cluster or central treatment. EDS can be a wholly gravity system (GEDS) or wholly pumped (PEDS), or a combination of both. The EDS approach can offer significant economies in reticulation costs and overall scheme costs compared to conventional sewerage, particularly in retrofitting sewer lines into unsewered smaller communities in difficult topography.
The components of an EDS scheme include:
- retention of existing (or provision of new) septic tanks on each lot
- low-diameter modified sewer lines (75 mm) for collection of septic tank effluent, special odour-venting controls on the lines, and pump units designed for septic effluent handling
- a modified cluster or central wastewater treatment plant designed to handle inflow of septic effluent, of reduced size due to the input of partially treated wastewater (the primary effluent from the septic tanks)
- a centralised operation and management system that oversees septic tank maintenance as well as treatment plant and final effluent land disposal.
Several EDS schemes are operating in New Zealand, producing significant economies in initial construction cost when sewering existing communities with on-site effluent management problems. However, when total maintenance costs based on a nominal septic tank pump-out frequency of three years are factored into a 20-year capitalisation of costs, in many cases this gives an economic advantage to MCS. Where the condition of existing septic tanks in a community requires substantial upgrade or renewal, the costs of the EDS scheme swings preference to the MCS. EDS has significant saving in terms of the community component of scheme costs when septic tank upgrade costs are left to lie where they fall (with each property owner). In this case, only the EDS system (including GEDS and PEDS elements) plus the scaled-down cluster or central treatment plant(s) comprise the publicly funded scheme. This benefits property owners with newer septic tanks, and disadvantages those with older tanks.
Some local authorities have difficulty envisaging an appropriate management process when some parts of the scheme are on private property (the primary treatment in septic tanks) and the rest is in community sewer lines and treatment plant. However, this can be readily overcome with co-operation from the community in allowing access rights for maintenance personnel. An important advantage of EDS is provided by a significant reduction in infiltration flows.
Modified effluent drainage servicing (MEDS)
This is a small-bore version of EDS for carrying filtered septic tank effluent from each property to cluster or central treatment. It is based on VGS (variable-grade sewer) technology out of the US. Each on-lot septic tank is an improved septic tank with a large-capacity single chamber and fitted with an effluent outlet filter. The solids control provided by the effluent outlet filter enables 30 mm on-property collection lines to pick up the septic tank effluent and transfer it to 50 mm public sewer lines, which increase to 75 mm as more properties connect. The lines can be installed by continuous-shallow-trenching machines at constant depth and following the natural lie of the land, thus substantially reducing construction costs.
Special design precautions are needed to deal with odour control, and pump-station and sewer-line maintenance. Sewer lines can flow uphill as required, as long as properties connected in the vicinity of uphill sections are elevated above the hydraulic grade line (HGL). Where properties are below the HGL, an effluent pump can be installed.
The MEDS approach to decentralised wastewater management for smaller communities can benefit both existing communities and new development. To deal with infiltration impacts on the treatment process, MEDS eliminates infiltration by providing a totally sealed system from the on-property improved septic tanks to the treatment plant.
Alternative sewerage experience in New Zealand
During 1995 a New Zealand-wide survey of city and district councils was carried out as part of a research project at the University of Auckland to obtain information on alternative sewerage schemes. This identified 14 operating schemes, encompassing three collection alternatives:
- EDS (effluent drainage servicing), where septic tank effluent is reticulated via 75 mm sewer lines for off-site treatment and disposal
- MEDS (modified EDS), where 50 mm variable-gradient sewer lines are used
- PEDS (pumped EDS), where everything is pumped into a pressurised reticulation system.
The carriage of anaerobic septic tank effluent off-site via sewers immediately suggests the possibility of odours. However, these were evident at only one scheme, a holiday settlement served by a PEDS system, where at the beginning of a holiday period odours were released from the on-property pump sumps if the lids were lifted for maintenance. Generally all operators were satisfied that odour and corrosion were not a concern. Maintenance problems in the sewer lines were a feature of some schemes where the homeowner was responsible for septic tank maintenance. In all cases where the local councils managed the total system, including on-property septic tanks, such problems did not occur.
Treatment of the reticulated septic tank effluent was best achieved by oxidation ponds or wetlands. These could be purpose-built as a cluster treatment plant, or the effluent could be transferred to an existing community treatment plant. Mechanical aeration plants based on the activated sludge principle were not entirely satisfactory, because the lower organic strength of septic tank effluent resulted in operating problems and poor performance.
Overall, scheme costs showed variable savings relative to conventional sewerage schemes, with PEDS systems showing greater savings (35–45%) compared to EDS (12–40%). However, costings were very site-specific, with alternative sewerage offering particular advantages in locations where very difficult topography and soil (such as rock) conditions make conventional sewers expensive. Some councils saved money on the community portion of the scheme by leaving the costs of septic tank upgrades on each property to lie where they fell – with the home owner.
Pressure sewers provide full off-site transfer of all household wastewaters by injection of grinder-pumped wastewater flows into a pressurised reticulation network (rather like water reticulation in reverse). The reticulation system can readily follow the natural ground profile at a shallow depth, including undulating and steep terrain, and can be directed around or over topographical obstacles. Effluent is anaerobic (no air surface is present as in a gravity sewer system), and special venting controls are required at the treatment plant discharge point. Maintenance oversight requires ready access to on-property pump units. Infiltration is eliminated.
Vacuum sewers can operate in conjunction with vacuum toilets (with very low flush) or normal low-flush toilets, and can pick up all other household wastewater flows for vacuum conveyance. They are most suited to flat topography, and are very useful in high-water-table locations such as around lake edges or along coastal strips. Vacuum lines have to be provided with regular low points (or transportation pockets) to facilitate plug flow between dwelling vacuum holding tanks and central collecting tanks. Because flow is continuously mixed with air it does not remain anaerobic. Infiltration is eliminated.
St Arnaud Village, Nelson Lakes, Tasman District
St Arnaud village is a holiday settlement on the shores of Lake Rotoiti, in Nelson Lakes National Park. Houses are located on rocky sub-soils, which not only creates problems for disposing of septic tank effluent but also makes construction of a sewerage system for off-site treatment of wastewater very costly. A joint working party was set up by Tasman District Council and the St Arnaud Community Association in 1994 to investigate alternatives for improved servicing.
Eight technical options and two management options were identified. The technical options were (1) the status quo, but with 'failed' systems replaced; (2) localised upgrades; (3) upgrade all systems to modern standards; and (4) use MEDS (modified effluent drainage servicing with 50 mm sewer lines and effluent outlet filters on septic tanks) in three clusters to collect septic tank effluent from problem areas. Each cluster treatment system consisted of sand-filter units, the resulting high-quality effluent to be drip-irrigated at low loading rates into natural forest. Satisfactorily performing on-site systems could connect to the MEDS lines in the future. Option (5) was to require all properties to connect into the three cluster systems.
Options (6) and (7) retained upgraded septic tanks on each property, but reticulated the effluent to centralised treatment via either MEDS 50 mm lines or EDS 75 mm sewer lines. Option (8) was for MCS (modified conventional sewerage) to centralised treatment and land application. The two management options were either council operation and maintenance oversight, or a community management district approach under body corporate control.
Initial cost comparisons showed an advantage to the full MEDS and cluster treatment approach. However, this relied on portions of the forest in the National Park being made available for low-rate drip irrigation of high-quality effluent. Because this did not prove to be acceptable, centralised treatment outside the National Park was required. At this point the cost comparisons favoured MCS and centralised oxidation pond treatment, which also had the advantage of being able to handle future subdivision growth near the village. The resulting scheme is under council operational management.
Conventional sewers versus alternative sewerage
One of the major problems with conventional sewer systems is the level of infiltration that occurs due to groundwater and surface water flows leaking into the sewer system during wet weather. It is almost impossible to eliminate this problem as manholes used for maintenance create points of potential leakage in the sewerage system unless the lids are sealed and bolted to the frame. On the other hand, many of the alternative collection systems referred to above for use in small-scale servicing systems enable fully sealed pipes with secure access and inspection points to be constructed so as to eliminate infiltration.