How we currently treat our wastewater
We’re proud to give our residents a great wastewater service. Our plant was upgraded in 2006 and we constantly look for ways we can improve our processes from both a sustainability and financial perspective.
Every year, up to 12.9 billion litres of wastewater is collected from 31,700 connections around the city.
Forty pumping stations help move the wastewater along a network of pipes that, if laid on end, would stretch from Palmerston North to Hamilton (405km).
When it arrives at our treatment plant in Tōtara Road the first thing we do is remove things that shouldn’t have gone down a drain – like wetwipes, condoms and tampons. These need to go in your bin.
The wastewater is then pumped with air to remove grit or sand, before passing onto our sedimentation tanks. Here, the solid material sinks and the cleaner material moves on. Solid material isn’t just poos – it includes fat and oil and food scraps from your sink.
It then goes to lagoons where air is pumped into the water allowing microorganisms to grow and eat any remaining solid material.
Then we remove phosphorus from the water and blast it with UV light. By the time the water reaches our small wetland, 99.9% of bacteria has been removed. The water then passes through the wetland into the river after about four days of treatment.
The remaining solids stay on site for 20 days, where bacteria help break it down before it’s removed offsite.
What are the contaminants we are treating?
Understanding the types and quantities of contaminants present in our wastewater helps us predict the requirements of the treatment plant in the future. Improvements to accommodate increased wastewater flows and changing contaminant loads will be a key part of the capital investment and work programme resulting from our future option.
Wastewater contaminants originate from residential, commercial and trade waste sources.
Wastewater treatment addresses four major groups of contaminants.
These are undissolved inorganic and organic materials that are suspended in the wastewater and pass through the initial screening process. The main two items are faeces and food. Some of the solids are removed through our sedimentation tanks where they sink to the bottom and are removed for further treatment in our aerobic digester tank. The remaining suspended solids are removed in our lagoons where natural microorganisms and bacteria consume them and transform them into biosolids. A small portion of solids remain in the discharge.
Dissolved contaminants or nutrients
These items include nutrients like phosphorous and nitrogen, and also a range of other contaminants such as metals and inorganic and organic compounds.
Phosphates come from cleaning products, fertilisers and detergents. Nitrates come from protein waste matter and ammonia in urine. These nutrients can contribute to algal blooms in oceans and rivers which overwhelm other plant and animal inhabitants in these environments.
Phosphorus and nitrogen are currently treated and removed in part from the wastewater by biological nutrient removal. At low river flows phosphorus is removed to very low levels by using chemical treatment.
This group includes viruses and bacteria. We measure faecal coliforms as an indicator of the presence of human or animal wastes in the water and while most are harmless, some can cause diseases, such as hepatitis B, gastroenteritis or typhoid.
Micro-organisms are removed at the treatment plant through physical settlement both with and without the use of chemicals while micro-organisms are inactivated using oxidation and UV light which can kill up to 99.9% of these.
Emerging organic contaminants
Emerging organic contaminants (EOCs) are synthetic or naturally occurring chemicals that accumulate in the environment with potential to cause adverse ecological and/or human health effects.
This is an area of ongoing research. Effects of the accumulation of EOCs in the environment are only starting to be understood as detection methods have only recently become available.
Examples include: food additives, pharmaceuticals, and natural and synthetic hormones. The extent to which EOCs are removed through the treatment process is specific to each chemical and the level of treatment. Membrane filtration systems can remove high levels of EOCs.
The trade waste factor
Around 12 per cent of wastewater volume comes from trade waste. It contributes varying amounts of contaminant loads depending on which contaminant (BOD 25 per cent, suspended solids 7 per cent, nitrogen and phosphorus 10-12 per cent).
Trade waste discharge is controlled through a permit system, and the low concentration of contaminant in trade waste discharge is due to pre-treatment onsite before it enters our wastewater system.
By comparison, residential and other commercial sources make up 88 per cent of wastewater volume and 75-98 per cent of contaminant loads.
Treatment under the three discharge options
Treatment options have been developed to allow Council to consider a total treatment scheme. This means the impact of different treatments from the moment wastewater enters our treatment plant, all through the process until it's discharged.
The options have been developed to include scientific investigations into the sensitivity of river flow levels and engineering expertise in treatment technologies that are used in New Zealand and overseas. We’ve also included wetland and/or land passage options prior to discharging to the river. We’ve analysed soil conditions to give us an idea of what kind of soils would be best for land application of wastewater and to determine the sustainable application rates for both nutrient and water.
Conveyance (pipes and pump stations) is also an essential part of many of the treatment options. For all options we have considered the lengths of pipeline lengths and number of pump stations required.
Staging possibilities and regional or sub-regional scheme options could allow us to pick up wastewater from communities in Marton, Feilding, Bulls and Halcombe, for example. Our options have identified the flexibility within each option to consider these.
The costs of these options have been developed considering all of these factors, forming the total scheme approach.
Discharge Option 1
Majority of treated wastewater is discharged to the Manawatū River via a wetland flow, with significantly improved removal of contaminants including phosphorus and nitrogen.
A five-stage Bardenpho Biological Nutrient Removal with Membrane treatment (MBR) and UV disinfection is the recommended treatment proposed for Option 1. The MBR Treatment Plant has the highest level of treatment proposed of all the options and would be one of the highest levels of treatment for wastewater currently used in New Zealand. This treatment process:
- Achieves the highest level of treatment before discharging to the river. The highest level of treatment is achieved when river flows are at their lowest and the river bed is more susceptible to adverse effects from the treated wastewater discharge
- Provides for removal of contaminants including nitrogen and phosphorus
- Can be staged to accommodate increasing growth and/or a regional solution
- Is relatively more affordable than other alternative options being considered
- A wetland and/or land passage is included before discharging into the river.
A Reverse Osmosis (RO) Plant was considered early on in the project evaluation process. This is a method used for treating water to drinking water quality. The RO plant is not proposed for this option on the basis it has high capital investment, operational and maintenance costs and the treatment process produces a brine product that is difficult to dispose of. The RO is also not well tested at the scale needed to treat Palmy’s volumes of wastewater.
Treatment upgrades will be required to the existing treatment plant. If a regional scheme is considered desirable, further upgrades to the treatment plant will be needed.
Option 1, which sees the majority of treated wastewater discharged to the river, includes a discharge to land during low river flow. This option will ensure all targets in One Plan are met at all times. This will require an approximate land area of 670ha (the smallest area of land compared to other options). This land may be accommodated within the Council's boundary.
Hybrid discharge between land (55%) and the Manawatū River (45%).
Option 2 will remove 55 per cent of the wastewater discharge from the river. The area of land necessary to discharge treated wastewater 55 per cent of the time has been determined through the modelling of river flows and the application rates of the treated wastewater to either fluvial soils or coastal sands.
Upgrades are needed to the existing treatment plant. We’d need to implement a number of new or improved process within the existing wastewater treatment plant. The outcome of these upgrades, results in a slightly higher quality of the treated wastewater compared to the existing wastewater discharge.
Significant land area is required, approximately 1,740ha of fluvial soils would be needed to discharge treated wastewater to 55 per cent of the time and this land is not likely to be available within the Councils boundary. If a regional scheme is proposed, additional upgrades to the treatment plant will be required in addition to acquiring more land.
All treated wastewater would be discharged via an outfall pipe located on the sea bed, and extending approximately 2km off the coastline (in ocean waters). The end of the outfall would be at least 20m below the surface of the ocean.
The level of treatment proposed for this option is similar to the existing wastewater treatment process however without current chemical treatment will no longer treat for phosphorus, as the sea water mitigates the impacts of this. This means only minor upgrades will be needed at the existing wastewater treatment plant.
In addition to upgrades to the existing treatment plant, new infrastructure is needed to pipe the treated wastewater to the outfall. This pipeline will be at least 1300mm in diameter and 38km in length, supported with at least 4 pump stations and an outfall of 2km in length and diffuser at the end.
A wetland and/or land passage may need to be considered for this option, prior to discharging via the outfall. This is not included in the option at this stage.
If a regional scheme is proposed, a larger pipeline and additional upgrades to the treatment plant, will be required. The discharge will be outside the Council's boundary.
As technology progresses and legislation changes, we can continue to improve treatment of our wastewater. A good analogy is windows. In the past many homes had single glazed windows, but over time people have upgraded to double-glazed. This is similar to how treatment could work. We could improve current processes, or if new technology comes in, we can add that into the process.