About wastewater

Pipes and pump stations convey wastewater (sewage) from homes and businesses to treatment plants, the main one in Bromley treats wastewater from Christchurch City and the Lyttleton Harbor, the remaining four are located on Banks Peninsula.

Christchurch City 

The City of Christchurch was built on low, flat, water-logged land and with no sewage system and cesspits for the wastewater. In its early days Christchurch was particularly beset with the problem of water-borne diseases such as dysentery and typhoid.

In 1874 it had the highest death rate of any centre in New Zealand. 

The original Number One Pump Station on the corner of Tuam St and Mathesons Rd is still standing (minus its smoke stack) and is a privately owned demolition sale yard.

With this as a background, in 1875 the Christchurch Drainage Board was formed and construction on Christchurch’s first pump station and first piped network was started in the late 1870s.

Now a vast network of 2,679 kilometres of public wastewater pipes, 30,817 manholes and 239 pumping, lift and vacuum stations continuously convey wastewater to the Christchurch Wastewater Treatment Plant for treatment and disposal.

Compared to other New Zealand cities, Christchurch is very flat with unique environmental features, and has its own particular wastewater network challenges. Pumping is required on the plains as the land does not slope enough for gravity alone to naturally move the wastewater load along. For more information on the Christchurch wastewater system, read Christchurch - Swamp to City - A Short History of the Christchurch Drainage Board 1875-1989.(external link)

It is important that wastewater gets to the treatment plant as soon as possible to avoid odour generated from its decay. The length of time wastewater takes will depend on the distance from the treatment plant (one to 24 hours maximum).

Christchurch has a particular problem with this as it has one of the flattest wastewater networks in the world. Where the volume of wastewater is not sufficient to cause a rapid flow, or where the pipe grades are too flat, it is sometimes necessary to flush extra water down line through the pipes to move wastewater along.

Bark biofilters are used in 33 locations to treat odours from the wastewater network.

The Christchurch Wastewater Treatment Plant at Bromley treats the wastewater from all of urban Christchurch (as well as Tai Tapu and the Lyttelton Harbour). The average daily flow is 185,000,000 litres per day. The highly treated wastewater is discharged through a 1.8 metre diameter pipe which has its outlet 3 kilometres into the Pacific Ocean off South New Brighton.

Banks Peninsula

On Banks Peninsula the Council has wastewater treatment plants at:

The Akaroa Harbour treatment plants have to cope with the wastewater from large numbers of tourists so the type and amount of wastewater that arrives at the treatment plants can change quite dramatically from winter to summer and even from weekday to weekend or on busy public holidays. This can cause variations in the treatment process.

The standard of the treated wastewater is monitored by the Council and Environment Canterbury to meet the resource consent conditions.

Wastewater treatment is the process of taking raw wastewater, and converting it through a variety of treatments into a final effluent that is without the solids and as many of the harmful characteristics of the original wastewater as possible.

There are a variety of different treatments used and available around the world and New Zealand, as there are in the wastewater treatment plants in Christchurch and Banks Peninsula.

They are all quite different in the way they treat but follow the general treatment practice of preliminary, primary, secondary and tertiary treatment. Biosolids are produced as a result of the treatment process and these are all taken to the Christchurch Wastewater Treatment Plant.

The treatment process

• Preliminary treatment - removal of floating material, heavy settleable, and large suspended solids, using structures like screens, grit chambers and flow equalization.
• Primary treatment - the removal of organic solids using sedimentation in primary sedimentation tanks.
• Secondary treatment - aerobic, or anaerobic biological decomposition of organic matter using such things as intermittent sand filters, trickling filters, aeration tanks and imhoff tanks.
• Tertiary treatment - removal of the remaining organic matter and killing pathogens (bacteria, viruses and protozoa) by disinfection with maturation ponds or ultraviolet (UV) light.

Most of the treatment plants that we operate currently discharge to the ocean and harbours, but there are two that discharge to land (Wainui and Tikao Bay). The treatment plants are upgraded when required to improve the final effluent quality and minimise the environmental effects.

The treatment plants are monitored by us and by the Regional Council (Environment Canterbury) to make sure that they meet the environmental measures set under their resource consents.

A project to remove the treated wastewater discharges from Lyttelton Harbour and instead to pump the wastewater from Lyttelton, Governors Bay and Diamond Harbour Christchurch Wastewater Treatment Plant was completed in 2023.

Domestic users are by far the biggest generators of wastewater due to the wide variety of uses of water in the home. The kitchen, bathroom, and toilet are the heaviest users of water, with toilet flushing consuming the most water.

Rainwater downpipe from gutter illegally connected to gully trap which allows rainwater into the wastewater system.

There are things you can do to help keep the wastewater system running smoothly from your house or business to the treatment plant where it gets treated.

Check your downpipes

You can help ensure the wastewater system runs efficiently by checking that your house roof downpipes are not being fed into a stormwater drain, not a gully trap.

When rainwater gets into the wastewater system from roof drain pipes it can quickly fill it up and prevent it from working properly.

Proper installation will ensure that your stormwater will not get into the wastewater network, which can cause overflows of untreated wastewater (sewage) during times of heavy rain.

This will avoid raw sewage waste flowing into rivers and spilling onto streets and private property.

Gully traps are the fittings that you can see outside bathrooms, toilets and kitchens that take wastewater from these rooms. 

Check your gully trap is not draining the area around it

Gully traps that are not set up correctly can allow stormwater runoff to enter them and into the wastewater system where it shouldn’t be.

The more of these that are like this at our houses and businesses, the more stormwater gets into our wastewater network causing problems like wastewater backing up onto properties and overflows to the environment.

Check that your gully trap isn’t in a low spot or flush with the ground surrounding it, if it is it may allow stormwater in during rain. To comply with the Building Code, it should be at least 25 mm above the surrounding ground level if the surrounding area is paved, or 100 mm above the ground level if it is unpaved.

There are gully trap surrounds that you can purchase from hardware stores, you just need to make sure you have the right size surround for your trap and that you seal it up to the ground and house so water can’t sneak past it. Alternatively, you could talk to a drainlayer about raising the trap up.

Before and after pictures of a low lying gully trap that could allow rainwater to be drained into the sewer system and upgraded with a new gully trap surround and sealed up to prevent inflow.

Avoid waste disposal units

Another way you can help the system work more effectively is by avoiding installing waste disposal units or waste grinders where possible, as these put an extra load on the wastewater system with the volume of organic matter requiring decomposition.

It is better to compost kitchen wastes or put them in your green organics bin. 

Minimise the fat that goes down the drain

Fat is the single biggest contributor to wastewater pipe blockages and dry weather overflows. It blocks household laterals and Council wastewater pipes and can be costly to unblock and clean up.

Fat is often poured down the drain while it is in its liquid form but then starts to harden as it cools while travelling down it. Over time the fat layer becomes thicker and thicker, slowly clogging the pipe like fat in an artery.

Instead of pouring fat down the sink, you should put pans with fat aside and allow them to harden, then scrape it out, or soak it up with paper towels and put in the green organics bin.

Businesses like restaurants have grease traps to capture this fat, which is cleared out regularly to minimise fat entering the wastewater network.

Minimise your use during rain

When rain is persistent and heavy over a long period of time the wastewater network can start filling up and eventually be overcome. At this point, wastewater can back up onto properties and overflows can occur.

Like all water authorities around the world, we have a problem with our wastewater systems called inflow and infiltration (I&I). This is when rain (stormwater) and groundwater makes its way into the wastewater network where it shouldn’t be.

Inflow is when rain gets in directly, like when roof downpipes are incorrectly drained into gully traps, or where gully traps are in low spots prone to flooding.

Infiltration is where groundwater leaks into buried wastewater pipes through joints or cracks.

I&I is most likely to happen during the peak wastewater use periods between 6am and 8:30am and between 4:30pm and 6:30pm (weekend morning peaks tend to be a little later).

If you can avoid doing things like putting the washing on, or having a shower or bath during these times when there is prolonged rain, it will minimise the chance of overflows occurring.

A good rule of thumb is, that if there has been more than 30mm of rainfall in the previous 12 hours, then minimise your water use during peak periods.

We have an ongoing programme of investigations and physical works to reduce the problem of I&I in our wastewater network.

Industry responsibility

Industry can also assist by adopting cleaner production methods (waste minimisation) as well as onsite treatment of by-products where warranted. These measures can have a significant environmental benefit as many firms use large amounts of chemicals in their manufacturing process and the resulting by-products are then introduced into the wastewater system.

If you want to look into how your business might be able to improve its wastewater discharge or have ideas you would like to check, please call the Council to talk them over with the Trade Waste team who are keen on helping businesses to do their best.

General

Pipework

Pumping

Treatment

The Council has obligations under specific acts and bylaws to safeguard the health of the community and the environment.

Wastewater regulation

The main regulations governing the collection, treatment, and discharge of wastewater are:

• Resource Management Act 1991(external link)
• Local Government Act 1974(external link) and Local Government Act 2002(external link)
• Water Supply and Wastewater Bylaw 2022
• Building Act 2004(external link)
• Trade Waste Bylaw 2015.

Monitoring of levels of certain chemicals takes place. In some instances particular substances are either prohibited, e.g. asbestos or organochlorine pesticides, or are allowed into the sewer only in limited quantities, as is the case with chromium, nickel and cadmium.