Wastewater

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.

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 Nature Calls project.

Wastewater contaminants originate from residential, commercial and trade waste sources.

Wastewater treatment addresses four major groups of contaminants.

Suspended solids

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.

Micro-organisms

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.

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.

Inflow and infiltration refers to rainwater and groundwater that gets into the wastewater pipes from broken sewer mains and house connections and from improperly connected stormwater systems. A common point of entry for stormwater into the wastewater network is through a gully trap. Council is actively trying to reduce inflow and infiltration.

We have a bio-gas generator located at the Totara Road Wastewater Treatment Plant, which supplies power to the treatment plant and Awapuni Resource Recovery Centre.

Methane gas is captured from the retired Awapuni Landfill – decommissioned in 2001 – and from sludge digesters at the wastewater plant. The digestors use a biological process to break down sewage sludge. The process is designed to capture the bi-product – methane gas – before feeding the gas into the generator as a fuel source to produce electricity.

Not only does this prevent methane gas from polluting the environment, but the electricity produced offsets about 40-50 per cent of the total annual power requirements for our wastewater plant.

In the year to March 2021, the bio-gas generator made 1,992 Megawatts an hour, while consumption at the treatment plant was 1,885MWh. That means 107MWh was sold back to the grid.

Since February 2018, we have made $11,600 from selling electricity back to the national grid.