The main purpose of a wastewater network is to protect public health and prevent people from coming into contact with untreated wastewater.
Constructed wastewater overflows to waterways act as safety valves on the wastewater network, so that when the network is overwhelmed in a large storm, wastewater does not overflow onto people’s properties or on the street.
CRC224552 Consent (external link)allowing the discharge of untreated wastewater to waterways was granted by ECan in 2016 and expires in 2029. In 2015 Council commissioned an assessment of the human health effects arising from overflow events [PDF, 131 KB].
The Council is compliant with this consent apart from one overflow location (corner of Sandwich Road and Eastern Terrace), which computer modelling shows overflows more often than the allowed twice per year. Projects to increase the capacity of the wastewater network in the area and reduce the frequency of this overflow are included in the Council's Long Term Plan.
The Council maintains a list of recent and historic overflows.
We all want to improve the quality of our waterways and we need to work out the best way to do this. We might get better outcomes by reducing contaminants in stormwater, rather than spending a lot of money reducing wastewater overflows.
For example, eliminating wastewater overflows altogether would require converting most of the city to a pressure sewer system and would cost more than $3 billion to build. However, it would not necessarily result in an improvement in water quality.
The Council carries out extensive monitoring of the water quality in our waterways and in the past year has collected and analysed over 7,000 water quality samples.
The key contaminants were:
- Sediment – the main contributor to poor water quality, which smothers habitat and food and can be contaminated. This comes from runoff from construction sites, unstabilised surfaces, vehicles and earthquakes.
- Metals (zinc and copper) – toxic to fish and other creatures. These come mainly from brake pads, tyres and building products (roofs, spouting and downpipes).
- Bacteria – create a public health risk. Bacteria mostly come from ducks and dogs and occasionally from wastewater overflows.
- Nutrients (nitrogen and phosphorus) – can be toxic (e.g. ammonia) and can encourage the growth of weeds and algae. Nutrients come from fertiliser, soils, and the faeces of livestock, dogs and ducks.
Most water quality monitoring sites show remained steady since 2007, with some sites worsening and some improving. Following the removal of large amounts of sediment, the waterways have been returned to pre-quake environments. The Council prepares a water quality monitoring report every year.
The Council has already done a lot to reduce wastewater overflows:
- Rubber flaps on constructed overflows stop river water from getting into our wastewater pipes during floods.
- In flood-prone areas, the vents on our manholes are being replaced with solid lids so stormwater ponding on the road does not get into wastewater pipes.
- In flood-prone areas plastic lids are being put on household gully traps so stormwater on private properties does not get into wastewater pipes.
- In Akaroa, Diamond Harbour and Lyttelton a house-to-house survey identified where property owners need to do repairs to stop stormwater from getting into our wastewater system.
- Completed the $150 million Major Sewer Upgrade project
- Completed the Riccarton Road wastewater upgrade.
Projects currently underway to reduce overflows are wastewater pipe upgrades at the southern end of Colombo Street and along Beckenham Street.
The Council's 2018 - 2028 Long Term Plan includes $10 million for four further projects to reduce overflows so that we fully comply with our resource consent. These are upgrades to the Eastern Terrace wastewater main, Tilford Street pump station and pressure main and the Opawa pump station, as well as a new pump station and pressure main from Somerfield to Saint Asaph Street.
Improving the quality of our waterways can’t be done by the Council alone – it needs to be done by the whole community. There are several things that you can do:
- Avoid flushing wet wipes down the toilet. They are not flushable, no matter what they say on the packaging. Flushing wet wipes and other items causes blockages and overflows.
- Put fat from cooking in the green organics bin rather than down the kitchen sink. Otherwise it congeals in the pipes and causes blockages and overflows.
- Do not feed ducks. They are a major source of faecal contamination in waterways.
- Pick up your dog's faeces and dispose of it in the red rubbish bin. This is a major source of faecal contamination in waterways.
- Wash your paint brushes in the laundry tub so that the wash water does not go through stormwater drains into waterways.
- Do not drop cigarette buts and other rubbish into drains and gutters.
- Choose roofing materials that don’t contaminate our waterways – non-steel products or new condition powder coated steel are best. Unpainted galvanised roofs are the biggest source of zinc in our waterways. Make sure galvanised steel roofs are painted and the paint is in good condition.
- Avoid copper spouting, downpipes and roofs. These are becoming a significant source of copper in waterways.
- Next time your car needs new brakepads, ask for copper-free brakepads.
- If you are building a new house, make sure that there are good erosion and sediment control measures in place to stop sediment washing off your property into waterways.