The wastewater treatment plant to be built as part of the Gisborne Wastewater Project, will use a process known as a biological trickling filter to treat the wastewater.
The process consists of large tanks filled with plastic media, sometimes shaped like petals, little wheels or as a lattice.
The plastic media are designed to have a high surface area and large air spaces. Human and other organic waste from the milliscreening plant is pumped to the top of the tanks where it is discharged via rotating arms on to the top surface of the media.
The wastewater trickles down through the large tanks where friendly micro organisms grow on the plastic media (like river rock slime), catching and treating waste as it passes through the system. The microorganisms use the waste as food allowing more microorganisms to grow. This process transforms human waste into a non-offensive biomass.
Over time the quantity of biomass builds up and the excess biomass starts to fall off. This is called "sloughing". The operation of the process is very simple. The quantity of biomass will grow to treat the amount of organic load in the wastewater. Flows are pumped to the top and gravitate down. Air is generally provided through natural ventilation with occasional fan assistance.
There is no need for advanced control and instrumentation, mechanical mixers, and aeration systems. Another major advantage is that, because the sludge is not separated in Stage 1, there are no sludge transportation or disposal costs.
In Stage 2 clarification, the treated effluent gravitates to large settling tanks. The excess biomass that has sloughed off the media settles to the bottom of the tank where it is removed as sludge. The clarified effluent spills over the top weirs and gravitates to the final effluent pump station. During periods of low flow some of the effluent is recycled back to the BTF to stop the biomass from drying out.
Trickling filters remove organic matter from wastewater. The trickling filter is an aerobic treatment system that uses naturally occurring micro-organisms attached to a medium (often plastic) to remove organic matter from wastewater.
The population of microorganisms attaches to the plastic surface as a slime layer. As the layer thickens (with microbial growth), oxygen cannot penetrate to the face of the plastic, and anaerobic organisms develop. As the biological film continues to grow, the microorganisms next to the surface lose their ability to cling to the media, and a portion of the slime layer falls off the filter.
This is known as sloughing and the sloughed solids are washed through by the wastewater to the underdrain system. This cycle of growth and sloughing is continuously repeated.
Treated wastewater is pumped from the BTF plant to the start of the outfall at Stanley Road and then to sea via the 1.8km outfall pipeline.
Trickling filter systems have been around for a long time. The earliest systems used between the 1900s and 1950s used stones as the medium over which wastewater trickled. Today, specially designed plastic media are used in place of stones. This is because they can provide many times more surface area in a much smaller volume.
Traditionally, the BTF process is preceded by a primary sedimentation stage and followed by a secondary sedimentation stage.
Both stages produce sludge which must be dewatered and transported to landfill or disposed of elsewhere.
Rather than installing primary settlement tanks and having an ongoing operational cost, the filters are to be loaded at a very low rate.
The organic material that would normally be separated in the primary tank will instead be transformed in the BTF filters. During Stage 2 secondary clarifiers will be installed to remove the solids produced in the BTF.
Deciding the best wastewater treatment process for Gisborne has been a long process. Council launched its Wastewater Strategy in 2002. The upgrades proposed included primary sedimentation by 2010 and high-rate activated sludge with ultraviolet disinfection by 2016. The Environment Court was dissatisfied with this strategy in that it had not sufficiently taken into account Maori sensitivities on the issue. The suite of resource consents applied for in September 2005 included a "boulder bed" to address cultural concerns but some submitters saw this as merely a token gesture.
Council began a series of informal meetings with key submitters to identify some common ground. They decided to inspect other wastewater treatment processes including the Hastings BTF trial, in which tangata whenua were genuinely interested.
The Wastewater Adjournment Review Group then began investigating the BTF process to help find a solution. The BTF process satisfies some cultural concerns. The treated wastewater emitted from the plant is no longer considered to be human in character and, as a consequence, is inoffensive to tangata whenua and suitable for discharge through the long outfall. This technology offers an effective, environmentally acceptable solution at a lower cost.
The first stage involves the construction and commissioning of pre-treatment facilities, biological trickling filters, final effluent pumping station, an operations control/power centre and associated site works at Banks Street by December 2010.
The second stage will see the addition of two clarifiers enabling secondary settlement, sludge handling facilities and UV disinfection by between 2012 and 2014.