Managing stormwater and wastewater

Conveyancing 

In New Zealand, wastewater and stormwater conveyance is managed in a number of different ways. The majority of reticulated systems are composed of separate wastewater and stormwater networks, (termed separated networks), while others have a wastewater conveyance network, but stormwater is allowed to soak into the ground, where ground conditions support this. In a small number of older areas of some cities and towns – mostly dating around the early 20^th century – some networks were designed to collect and convey both wastewater and stormwater. These systems are called combined networks

Even in separated network areas, some stormwater and groundwater can get into wastewater networks. As a result, during periods of rainfall, the volume of stormwater can increase and exceed the capacity of a wastewater network to convey it. Because the network capacity can be exceeded, the network is designed to release heavily diluted wastewater into the environment. These types of overflows are termed “wet weather overflows”. Having designed overflow points within the network reduces the likelihood of overflows onto private property and helps to protect public health. ⁴⁸³⁶  

There are a number of ways to reduce the frequency and volume of wet weather overflows. Property owners should ensure downpipes are not connected to gully traps, and gully traps do not allow stormwater run-off from surrounding areas.

Another type of overflow is termed “dry weather overflows”. As the name suggests, this type of overflow does not involve increased volumes of stormwater getting into the network, but is caused instead by blockages caused by the build-up of fats or other materials in the network. These blockages can cause wastewater to overflow at various points within the network. It is important that fat, rags and wet wipes are not flushed down the sink or the toilet.

Wastewater systems typically use gravity to move wastewater discharged from houses and other premises along pipes towards the treatment works. When the wastewater pipes reach the lowest point on land, gravity will no longer move the material, and pumping stations are required to pump the wastewater up to the treatment plant. Many pumping stations are located near the coast, as this is often the lowest point to which wastewater can be moved by gravity.

Pumping stations will usually have some holding capacity for wastewater in the event that the pump fails, such as when there is an interruption in the electricity supply. When the pump stops for any reason, the wastewater will keep flowing as a result of gravity and it will build up in the holding tanks at the station. When the holding capacity of the station is exceeded, untreated wastewater typically flows into the sea or a nearby river.

Stormwater infiltration can also cause major problems in wastewater treatment plants by increasing the volume of effluent that they have to deal with, reducing the effectiveness of treatment, and increasing the risk that treatment systems will flood and allow effluent to escape. This is actually quite common as a result of faulty pipes, illegal cross connections (where private properties illegally attach stormwater drains to the public wastewater system. 

Another way in which untreated urban wastewater can enter the catchment is where sewer systems overflow into the stormwater network (exfiltration) and the natural environment. This generally happens as a result of failures in the pipes such as cracking followed by tree root invasion. It can also occur as a result of heavy rainfall. Pumped wastewater systems are becoming more common, and are resistant to infiltration, but failures can result in spills.

Case study: Auckland Region

The Auckland Isthmus has had on-going issues with wastewater overflows from the wastewater network, as well as discharges from poorly performing septic tanks. Water quality at a number of Auckland beaches exceeds the national guidelines on concentration of the bacteria enterococci. This bacteria indicates the presence of faecal material and disease -causing micro-organisms in coastal waters, high levels of which pose a significant risk to public health. In November 2011 more than a third of the region’s most popular beaches failed water quality tests and signs recommending no swimming were erected for at least 10 beaches. ⁴⁸³⁷  There are now permanent health warnings in place at Cox’s Bay, Meola Reef Weymouth Beach, and Wairau Outlet (which are reticulated for wastewater) and Little Oneroa Lagoon, near which a number of septic tanks are located. ⁴⁸³⁸

The growth and development predicted for the Auckland region, and the ageing infrastructure, means there is much concern about the capacity of the network in the future. Watercare Services is now focusing on delivering an $1 billion Central Interceptor Project which will see a large wastewater pipeline constructed to improve the wastewater network service for western and central Auckland and at the same time enabling growth in the east and south by reducing load in those areas. The proposed 13km long interceptor, which could be up to 13 kilometres long, will reduce wastewater overflows, provide for population growth and mitigate the risk of pipe failures. The interceptor will be constructed between 2019 and 2025 to transport wastewater from central and west Auckland to the Mangere Wastewater Treatment Plant.  Human effluent contains a number of substances that can be extremely damaging to the environment if released into the coastal or freshwater waters untreated. The most significant pollutants are pathogens and nutrients including nitrogen, phosphorous ammonia, organic matter and suspended solids.  

Most of the household wastewater in New Zealand is piped through treatment plants to remove pollutants before discharge. Most towns of 5,000 or more people (or residents) have a reticulated wastewater system which pumps everybody’s wastewater into a common treatment plant and discharge system. There are more than 250 treatment plants in New Zealand and although 24 of them discharge to land, the rest discharge either into the ocean, rivers, streams or drains. 

The plants vary in age and technological capability. Treatment plants need to be operated within flow and pollutant loads or concentrations that are specified in a discharge consent and this typically links with the technology type.

Wastewater discharge consents are granted and monitored by Regional Councils and can have a duration of up to 35 years. When a discharge consent is nearing expiry (within 6 months), a new consent application needs to be lodged to provide continuity of operation. The application needs to consider:

• the sensitivity of the receiving environment;
• what proven technologies are available;
• the financial implications of any associated upgrades;
• policy statements, district and regional planning documents;
• case law; and
• alternatives including technology options and discharge locations.   

The combination of the above is used to determine the best practicable option, which is reflected in the conditions in the consent, the plant type, the discharge system and the discharge location.

Treatment systems can be a basic oxidation pond system, or have a number of stages, including some or all of the following sequential stages, inlet screen, grit removal, primary sedimentation, secondary biological treatment and tertiary treatment.

A large number of small to medium communities in New Zealand have oxidation ponds. Oxidation ponds are large lagoons typically greater than 1m deep. Solids in the wastewater settle to the bottom of the pond and are gradually digested. Algae and bacteria break down organic matter and consume nutrients in the wastewater. Oxidation ponds have a low energy demand, however they require large amounts of land and the performance varies with the seasons.

Inlet screens and grit removal eliminates inorganic debris in the wastewater such as cotton buds, condoms, rags and stone chips from the roads. The items removed in this process are called screenings and are typically disposed of to landfill.

Primary sedimentation is a process that reduces the speed of the wastewater and allows settleable and suspended solids to drop out by gravity in a tank. Chemicals can be used to speed up the settling process.  The settled solids are called primary sludge.

Secondary biological treatment uses micro-organisms to break down organic matter and can also be configured to removed ammonia, nitrogen and phosphrous. Essential to secondary processes is having bacterial biomass that is grown under controlled conditions that consumes the nutrients from the wastewater. As the bacterial population grows from consuming nutrients, some biomass needs to be wasted from the system, when thickened this is called secondary sludge.

When a very high quality treated wastewater is desired, tertiary treatment is used to remove remaining substances. The most common form of tertiary treatment in New Zealand is ultraviolet (UV) disinfection to remove pathogens.

The primary sludge and secondary sludge when dewatered is called biosolids. Biosolids may be suitable for use as a soil additive (but this may add to pollution risks) or it may need to be disposed of in a landfill site.

Some of the challenges faced by wastewater treatment plants include:

• expanding plants to keep up with urban development;
• managing the variable flow and load in holiday destinations during peak periods; and
• managing the volume of stormwater entering the system. 

Septic tanks 

Smaller settlements in New Zealand often do not have community wastewater treatment plants but rely on individual household septic tanks. Septic tanks are generally effective but can fail where the soil is unsuitable, or where the surrounding land area is not big enough for the soakage field. This is a particular problem in modern coastal developments, where large homes (rather than traditional baches) with modern conveniences requiring a large amount of water, are built on small sections very close together. Overloading of the soakage fields, with resulting overland flow, is the most common problem. Septic tanks can also fail when they are inadequately maintained. Effluent that seeps untreated from septic tanks may get into streams or groundwater, and from there into the marine environment. Even with properly operating systems, some nitrogen transfer to groundwater is inevitable. 

See the Ministry for the Environment guidance document: Sustainable wastewater management: A handbook for smaller communities, for more information regarding community designed systems. 

Agricultural and industrial enterprises 

 Meatworks, dairy factories, tanneries, fertiliser plants, stockyards, mills and many other types of agricultural and industrial enterprise all risk pollutants being discharged into the catchment. In the early years of European settlement, there were many such enterprises scattered across the country, all producing industrial or agricultural waste on a small scale. Some of these were connected to municipal treatment plants while others managed their own waste. In some cases, toxic waste was discharged directly into the marine environment.

Over the last 30 years, agricultural and industrial activity has changed, so that there are fewer, larger operations. For example, whereas in 1970 there were more than 200 dairy factories, there are now only about 30. ⁴⁸⁵⁴

These bigger businesses tend to deal with waste more effectively because of their relative financial strength and access to technology. Furthermore, the last 30 years have seen the implementation of stricter legal requirements and a shift in attitudes towards greater recognition of the importance of avoiding pollution, together with a greater interest in finding better technology to achieve this.

Agricultural enterprises generally now use two-pond treatment systems, often combined with land irrigation systems, to dispose of their effluent from dairy sheds and the like. Such systems pass the effluent through two ponds, each containing different naturally occurring types of bacteria, before discharging it into the catchment. This system removes 95 percent of the organic matter from the effluent but may not greatly affect dissolved nutrients levels.

Land irrigation disposes of wastewater and nutrients to paddocks or forest rather than into the water. This is effective at protecting the catchment from some pollution, so long as the land is not overloaded. But if the capacity of the land to absorb the waste is exceeded, the nutrients will wash off into streams or leach into the groundwater, posing the same risk to marine ecosystems as they would have if they had been discharged directly into the catchment. 

Trade Waste 

In relation to the discharge of non-domestic wastewater into a municipal wastewater system, the term “trade waste” is used.  Trade waste means any liquid, with or without matter in suspension, that is, or may be discharged, from a trade premises to a wastewater system. The management of trade waste by councils is to protect the health and safety of people, the wastewater system itself, the environment, as well as to assist treatment plants to process wastewater and produce biosolids to meet any required standards. Management of trade waste is usually attained by the implementation of a trade waste bylaw.

Last updated at 1:09PM on February 11, 2018