Wastewater Treatment Plant
Biological Trickling Filter Plant
The Biological Trickling Filter (BTF) plant was built alongside the existing milliscreening plant at Awatoto in 2014. The wastewater treatment upgrade provides a secondary treatment process that includes grit removal followed by biological treatment. It is designed to allow for further treatment stages to be added in future if required.
After the effluent passes through the head works area it moves to the main pump station. This structure is comprised of four wet wells below ground and a large dry well where the mechanical pumps are housed. An above ground structure houses the electrical and process control equipment which runs the pump station.
The effluent is pumped to one of the two Biological Trickling Filters. These filters are 26m diameter concrete tanks and each tank is comprised of 32 concrete panels eleven metres high with a hardwood timber flooring system. The flooring supports 14 layers of welded plastic modules on which bacteria grow and feed on the effluent, transforming it into a non-offensive bacterial biomass.
A distributor arm sits atop a concrete central column in each tank. Each distributor has eight arms that sprinkle screened wastewater onto the plastic filter media in each of the two BTF tanks. Both tanks are fitted with aluminium dome roofs to contain and control any odour.
Each tank is fitted with a recirculation air system comprising of a fan outside the tank. Air is drawn from the network of pipes under the timber floor and is recirculated into the top of each tank. The tanks are also fitted with an extraction air system of two fans (operating on a duty/standby arrangement) that extract air also from under the hardwood floor. The foul air is delivered to the nearby bark bio-filters, which act as odour treatment beds.
As the final stage in the process, the treated water leaves the BTFs and flows through one of two Rakahore channels (an open channel filled with rocks) which provide spiritual cleansing before discharge via a 1.5km outfall into Hawke Bay at Awatoto.
Industrial effluent is also received at this plant but does not pass over the BTFs. A separate building houses rotating screens specifically for industrial waste flows and the domestic grit classifier equipment. This building is constructed from concrete pre-cast panels and aluminium panelled cladding. The western end of the building houses the process and electrical equipment for the plant and a computer server room sits in the upper level.
An overflow basin has been constructed south of the Rakahore Channels for the purpose of containment during an emergency. The basin has been grassed where the sides are able to be mowed and planted on the steeper sides for easy care.
Many of the structures have been treated with a fibreglass coating to reduce concrete damage that is caused by the corrosive nature of the effluent. All materials used in the plant were also selected to minimise the risk of corrosion in mind, both from effluent and the nearby coastal environment.
Biological treatment of wastewater is well established, both in New Zealand and overseas, and is relatively economical and simple to operate when compared to most other treatment processes. Among the major advantages of the BTF process is there are no sludge transportation or disposal costs. It also avoids the use of chemicals, which would eventually become part of the discharge.
Background and History
In August of 1887, 16 cases of typhoid fever were reported in the vicinity of Enfield Road. Poor sewage disposal was the contributing factor. This disease remained a danger in the region until the completion of swamp reclamation and drainage projects after the turn of the Century.
Between 1910 and 1915 Napier developed an improved sewage disposal system. The council adopted a system involving a number of ejector pumps, operated by air pressure generated at a central pumping station, to concentrate sewage at one outlet. This site, at the end of the Inner Harbour's Eastern Pier, became known as Perfume Point. Refuse not suitable for disposal by drainage was collected and taken to the refuse-destructor on the old Recreation Ground. Upgrades continued throughout the early 1900's. Napier depended upon a system of centrifugal pumps or pneumatic ejectors to raise the sewage from the flat areas of Napier to the outlet tanks.
On February 3, 1931, a major earthquake struck Napier. The earthquake raised the floor of the Inner Harbour and reduced the force of the tidal current. Sewage accumulated around the harbour entrance, infecting shellfish and cases of typhoid were contracted by some people who had eaten mussels taken from a reef about a half a kilometre from the sewer outfall.
The earthquake cut off electricity supply to the sewers, which had run from south to north, and were flattened or lowered by the uplift of land in the opposite direction. It was then decided to abandon nearly all sewers in the central business district and the flat areas. Some were converted to stormwater drains. The same general design was used for the new sewers but the pipes were laid under footpaths instead of under the centre of roads. In addition, some pumping stations were re-sited and some ejector stations were replaced by pumping stations.
In 1973, a new outfall with a comminutor station (to cut up solids) and pipeline were opened at Awatoto.
Napier City's sewerage collection, treatment and disposal system is limited to the city's boundaries. Meeanee, Jervoistown, residential Awatoto, parts of Bay View and Poraiti are not included in the system.
All the sewage in the serviced area is collected by a series of pipes to one of 44 pump stations and is then pumped to the Milliscreen Plant at Awatoto, which was constructed in 1991. The sewage passes through 1mm screens before being discharged into Hawke Bay via a 1.5 km marine outfall. The effluent is required to comply with the consent to discharge into Hawke Bay, as granted by the Hawke's Bay Regional Council (HBRC). The screened material is dewatered and disposed of at the Omarunui Landfill.
Construction for Napier’s Biological Trickling Filter (BFT) plant was completed and opened for operation in August 2014.