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Why wastewater discharges happen

Questions and answers

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Why discharges happen Q&A

Frequently asked questions about what causes the city's wastewater network to overload in heavy rain and why emergency discharges happen. Also what we're doing to help reduce the problem and what you can do to help.

The difference between wastewater and stormwater

What is wastewater and where does it go? 

Wastewater, commonly known as sewage, is the used water and waste from your home's kitchen, laundry, bathroom and toilet. It's also the used water from industry.

The wastewater from your property does down the drain, into pipes on your property. At your property boundary your pipe (lateral) joins into Council’s wastewater network of pipes, through pump stations to the wastewater treatment plant in Banks Street.

Council's network consists of 223km of mains pipes, 2806 manholes, 40 pump stations and the treatment plant.

We own and manage 50% of the wastewater network. The remaining 50% is owned and maintained by individual property owners. This has significant implications for managing the wastewater network in an integrated way. 

Sewer network diagram

What is stormwater? 

Stormwater is the rainfall that runs off hard surfaces such as your roof, driveway, the road, footpath, industrial yards and even the lawn.

Stormwater pipes and drains collect the rainfall, those pipes carry it untreated to the nearest waterway. By putting anything other than pure rainwater into stormwater pipes affects our environment.

Keep stormwater out of the wastewater system!  

The wastewater and stormwater systems are separate systems and it's the homeowner's responsibility to make sure they stay that way on their property.   

What causes emergency discharges in heavy rain?

During intense or heavy rainfall, some parts of Council’s wastewater network gets inundated with stormwater and the network can't cope with the volume of water going through it.

This can then cause wastewater overflows onto private property. Some households are not able to use their toilets and showers during heavy rain. 

When this happens, Council has to release the pressure in the system and discharge the wastewater into the river to reduce health risks to property owners from overflows. 

The overflows happen mainly because large volumes of stormwater gets into the wastewater network. Stormwater can get into the wastewater network from:  

  • roof water being piped straight into the wastewater system through gully traps or lateral wastewater pipes
  • lateral wastewater pipes from houses to the main network leaking
  • flood water (even minor) flowing over and into gully traps
  • water getting into cracked and leaking gully traps
  • Council’s wastewater system leaking at main wastewater pipes and access points.

It only takes 4 properties with downpipes going into the gully trap to overwhelm the sewer in that catchment. Just 2 properties with surface water flowing into their gully traps can do the same.

Why we discharge wastewater into the city’s rivers

We seriously consider all options before discharging diluted wastewater into our city rivers. 

The reason the emergency discharge happens is because an excessive amount of stormwater has entered the wastewater system.  A large amount gets into the wastewater system from illegal downpipe connections, from spouting into gully traps and surface flooding getting into gully traps on residential properties.

We only discharge when it's absolutely necessary and only in the area with issues.

We do this to prevent sewage from overflowing back into people's homes and on the roads through manholes.

The decision to open the valves is left until the absolute last minute when there's no other option.

What we do when MetService issue a heavy rain warning 

When MetService issue a heavy rain warning for Gisborne city, our contractors go out and check sewer manholes, pump stations and valves. This is to make sure they’re working as they should. They constantly monitor the network's hot spots, to help prevent overflows and discharges.

Our pump stations have alarms that tell the contractors when there’s a problem, like too much water. We’ll take measures like using big tanker trucks to pump water out of an overloaded area to waste trucks in an effort to avoid discharges.

But discharges will continue to happen if stormwater water keeps getting in from private properties. We require the cooperation of property owners in the city in order to reduce the frequency of discharges to the rivers.  

Who we notify when we discharge into our city's rivers

The discharge is highly diluted with rainwater, but there is a risk to health. We notify residents via our Facebook page, website. media and email a list of contacts.

We also notify the Medical Officer of Health, water users and sports groups.

Temporary warning signs are installed at swimming and recreation sites. 

We advise people not to swim or gather shellfish for 5 days after any heavy rain event, regardless of whether there has been a sewage discharge or not. 

Our Council is not unique when it comes to sewer overflow discharges to waterways. Other councils in New Zealand discharge wastewater into waterways when the sewer system is overloaded.

Process for notification when the scours are opened

Water Utilities notify our Pollution Hotline when the scours have been opened. 
Pollution officer phones Communication Team and Environmental Health (EH). 
EH sends out notification to the email distribution list and puts signs up. 
Communications post notification on the website and Facebook page. 
Water Utilities notify the Pollution Hotline when the scours are closed. 
Pollution Hotline advises Communications Team and EHO. 
Communications updates website and Facebook. 
EH notifies via the email distribution list, takes signs down 5 days after closing scours.

Number of discharges to waterways due to heavy rain since 2014

7 September 2018 - period of heavy rain 
6 August 2018 - rainfall 79mm
11 June 2018 - rainfall 113mm over 7 days
7 June 2018 - Kaiti area inundated
4 June 2018 - intense heavy rainfall in 8hrs - 59mm 
3 September 2017 - rainfall over 48hrs - 76.6mm 
28 May 2017 - rainfall over 48hrs - 75.4mm 
12 May 2017 - Cyclone Donna rainfall over 48hrs - 52.4mm   
13 April 2017 - Cyclone Cook rainfall over 48hrs - 64mm  
4 April 2017 - Cyclone Debbie rainfall over 48hrs - 82.2mm 

Report to Assets & Infrastructure committee - weather related events update

September 2015, April 2014, June 2014, August 2014 

How many unintended overflows to waterways have occurred since 2014?

February 2016 - fat blockage overflow to Wainui Stream
April 2016 – fat blockage overflow to Waikanae Stream
November 2016 – fat blockage on Stout St 
March 2015 – mechanical fault at Steele Rd pump station overflow to Wainui Stream
December 2015 – equipment malfunction at Russell Street pump station
August 2014 – pump station blocked by grass clippings overflow to Taruheru River.

About the wastewater network

The city wastewater network services around 14,750 houses and businesses.

An average of 13,000 cubic metres of wastewater per day goes through the network to the treatment plant. That's about the volume of water in 8 Olympic pools.

The council maintained network is a big system - 223km of mains pipes. 2806 manholes, 40 pump stations and a wastewater treatment plant.

What we're doing to prevent overflows

DrainWise has been set up specifically to reduce wastewater overflows in residential properties and discharges into rivers.

Helping homeowners

DrainWise will focus on solutions for helping homeowners fix stormwater drainage on their property so we can reduce the risk of rainwater getting in to the network.

We're continuing a programme of inspections in the worst affected catchment areas like Kaiti and Whataupoko. We're using smoke testing to find where wastewater pipes are connected or not sealed. We can also use a camera to check the condition of pipes under your property.

We still need to know where stormwater is getting in, so we know what our options are for helping homeowners fix it. We also want to make sure rain can drain from your house and off your section to the stormwater network.

You can help us by completing the property flooding survey 

Council pipes

We’re also completing renewals of council pipes and backup systems and investing $22m to renew our infrastructure as set out in the Long Term Plan.

We’ve spent $8m in renewing infrastructure since 2012. This includes work to replace 100-year-old pipes, upgrading pump stations, sealing off cross connections between the wastewater and stormwater networks and installing emergency storage tanks like the two 45,000 litre tanks in Steele Road.

It isn’t as simple as increasing the size of Council’s main pipes. Increasing capacity would impact on our treatment plant infrastructure too, which shouldn’t have to process what was clean rain water. 

Why does it take so long to inspect all the properties?

With around 14,750 properties, it will take time to inspect them all - we have a 10 year programme to work our way around the city.

Most properties in the city have been inspected at some stage, but with the renewed DrainWise focus, we’re now investigating getting rainwater off people’s properties, as we’ve found there are multiple routes for water to get into the wastewater system from private property.

Previously our focus was on getting downpipes out of the gully traps, but this was only part of the problem.

Why fill in the flooding questionnaire, when you know about the problems?

Your answers on the questionnaire will helps us prioritise the areas to investigate and potentially invest in, but our budget limits us to addressing small areas at a time.

If you have issues on your property - please tell us on the questionnaire

Why are you only just looking at the private issues now?

Most of the city has previously been inspected over the years, but the DrainWise focus is now on getting rainwater off people’s properties, rather than just removing downpipes from gully traps. 

Depending on the age of the house, we’re finding 10-50% of gully traps in any one street are not water-tight and can let rainwater into the wastewater system. 

During inspections our DrainWise team are doing basic sealing of gully traps. But if the gully trap needs to be raised or renewed or we find problems where a plumber or drainlayer is required, the property owner is responsible to fix them. We give them 30 days to fix the issue, but will work with them where they cannot meet that.
Read property inspection Q&As 

What you can do to be DrainWise

Check the drains and pipes on your property. You may need to:

  • redirect your downpipe away from the gully trap
  • make sure your stormwater is drained into the Council stormwater network
  • increase the height of your gully trap
  • seal cracks in gully traps or wastewater pipes
  • for some larger problems you may need to install new lateral pipes or get a building permit. Talk to us before you start. We’ll help you get the right approvals
  • employ a qualified plumber or drainlayer.

If you have issues with surface water or your household plumbing when it rains – complete our survey

If your downpipe is directed to your gully trap, this this, you need to fix it now. 

Illegal downpipe connection

Conserve water during heavy rain:

During and after a heavy rain event, try to reduce flushing the toilet, and using your dishwasher, washing machine or draining the bath.

Why don’t we upgrade the wastewater system?

The city’s wastewater network has been designed and built to manage the wastewater needs of Gisborne households and businesses for growth over the next 30+ years.

What we're finding is that as private property wastewater systems degrade with age, more clean water flows can get into the wastewater system, by either groundwater or rainwater.

Our inspections help identify issues on properties, so over time, as property owners repair and replace their own pipes, these flows will start reducing.

We renew $1m of our wastewater pipes every year.  But this is not enough to replace all the pipes that currently need replacing. Wastewater pipes generally have a 100 year life.  

What we're aiming to do

Our aim is to meet the following levels of service:

♦ Reduce wastewater releases into rivers and streams from 4 times per year (average) to once every 2 years (average) by 2026/27.
♦ Reduce wastewater discharged onto private property during heavy rain from 4 times per year (average) to once every 2 years (average) by 2026/27.
♦ Greatly reduce the number of times households can’t use their toilets and wastewater system during a heavy rain event by 2036/37 (10-20% probability of occurring annually).
♦ Reduce ponded water on private properties causing dampness in households.

Areas of responsibility

Council’s responsibility starts at the property boundary.

Sewer pipe responsibility

The exception to this is where Council may need to be involved to reduce stormwater flooding on properties because it's getting into the wastewater system.

Flooding on one property may lead to wastewater overflows at other properties, usually low-lying neighbours.

This is because the whole wastewater network is interconnected and water will always flow downhill. Sometimes the person responsible for the problem is not the one that bears the negative impacts. 

Prioritising the work

With so much work to be do, it's important to prioritise the work. We'll prioritise based on reducing flows where the greatest amount of stormwater could enter the wastewater network.

Our wastewater and stormwater models will be used to identify these priority areas. Flow monitoring shows that Kaiti has the highest percentage of stormwater entering the wastewater network followed by Whataupoko / Mangapapa then Elgin/CBD.

Sewer priorities

For more information see our DrainWise project.

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