Introduction
Process Theory
General Plant Description

Inlet Structure
Pre-Treatment
Screenings
Grit Removal
Foul Air Fans
Raw Sewage Pumps
Bioreactor
Wasted Activated Sludge
Secondary Clarifiers
Process Building
Motor Control Room
Emergency Generator
R.A.S. Pumps
Aeration Blowers
Treated Effluent

Introduction

The Te Maunga wastewater treatment plant provides treatment for domestic, commercial and industrial communities from the Mount Maunganui and Papamoa catchments.

Originally the plant was designed as an extended aeration system to achieve ammonia removal through the nitrification and denitrification process. A recent upgrade of the biological process has seen the system converted to plug flow Ax-O2-Ax-O2 (anoxic-aerobic-anoxic-aerobic) process.

The mean daily flow is 10245¬¬m3/d, a population equivalent of 36500 people. The industrial contribution to the plant is not significant; 6% of the flow to the plant is estimated to be industrial and this contributes approximately 13% of the COD load

Terminolgy

 

Current plant INFLUENT organic and nutrient concentrations are:

Current plant EFFLUENT organic and nutrient concentrations:

 
Bioreactor ditch capacity

Top

Process Theory

The process of treating influent to the end point, is dependent on maintaining the correct balance of biomass in the bioreactor and supplying an adequate amount of air to meet the biological oxygen demand.

The objective of the anoxic-aerobic-anoxic-aerobic treatment process is to achieve BOD removal, suspended solids removal and nutrient removal in the form of nitrogen.

The activated sludge develops naturally in a controlled environment, forming into a floc which can easily separate to produce clear liquor, known as final effluent. This floc absorbs dissolved and colloidal pollutants from the incoming raw sewage; the micro-organisms within the biomass use this material as “food” to promote cell growth and reproduction. A supply of oxygen is provided by blowers to sustain the needs of the micro-organisms as they metabolise and reproduce.

The control variables for optimum plant performance are very limited, they are:

•  sludge wasting (W.A.S)
•  return activated sludge (R.A.S)
•  aeration control (Dissolved Oxygen)

The average time a micro-organism remains in the bioreactor before discharge is determined through the amount of sludge wasted. The amount of sludge wasted controls the sludge retention time (SRT) or sludge age.

SRT influences:

•  mixed liquor suspended solids (MLSS) or concentration of micro-organisms in the bioreactor
•  oxygen demand (increases endogenous respiration)
•  sludge settleability (SVI)
•  nitrogen removal

Top

General Plant Description

Inlet Structure

At the front entrance to the treatment plant there is a concrete inlet structure where influent from Mount Maunganui and Papamoa is received. Two pipes lead out of this chamber in the direction of the Pre-treatment building, although there are two pipes, only one is currently operational as the second pipe is for future increased flows.

Top 

Pre-treatment

Pre-treatment is generally the first stage of any treatment process once the influent has arrived at the treatment plant. It is the point at which objects such as rags, plastics, tins, wood, stones and grit are removed. Many of these foreign objects are likely to cause blockages, wear within pipes and generally be a nuisance factor within the plant.

Top 

Screening

Te Maunga has 3 Brackett Green CF100 Band Screens each with the capacity to handle 500 L/sec. The purpose of the screens is to remove inorganic material such as tins, toys, synthetic fabrics that arrive at the plant with the raw sewage. It’s amazing what is put down the sewage system that does not belong there.

The screens remove any material in the raw sewage larger than 3mm, the removed material is discharged into a screenings washer to smash and break down organic matter and wash out faecal matter. The washed material then goes through a screw compactor to compress and squeeze out any liquid. The purpose of removing the liquid is to reduce the volume and weigh of material going to landfill.

The compressed material is transferred from the compactor via conveyor to a fully enclosed transportable bin which is taken to landfill once full. Approximately 5 tonne of screenings goes to landfill every 3 days.

Top 

Grit Removal

There are two Pista Grit chambers for grit removal. The chamber has a peak maximum flow rate of 880 L/sec, which equates too 21000 m³/d, which is sufficient to remove grit for the current design flows.

The Pista Grit removal system uses a vacuum pump to prime the main suction pump. Once primed a pinch valve opens and pumps grit from the grit hopper to a grit classifier where the grit settles out, while water and floating organics are returned to the effluent stream. The grit particles are screw conveyed up an incline and discharged into the same bin as the compacted screenings. The slope of the conveyor allows water to flow back.

Top 

Foul Air Fans

There are two foul air extraction fans, a duty and standby. Odorous air is extracted from within the pre-treatment building, raw sewage inlet pump station and associated influent channels where it is force blown to an odour bed consisting of a soil and bark mixture.

The odour bed is approximately 600 mm deep with a 300 mm central pipe with lateral pipes at approximately 2 meter centres. The laterals have small holes along their length to allow air to flow up into the soil and bark mixture before dispersing to atmosphere. Micro-organisms living within the odour bed strip odorous compounds from the air.

As the odour bed needs to be kept moist there is an automatic sprinkler system (6 heads) set up around the perimeter of the odour bed. A sensor known as an irrometer is connected to the sprinkler controller system and determines when water is required.

The odour bed media requires replacement approximately every 8 years as over time the bark will break down and rot away. The bark allows the odour bed to be porous and provides a large surface area for micro-organisms to attach to.

Top

Raw Sewage Pumps

From the Pista Grit chamber influent flows through to the raw sewage wet well. Three relift pumps with variable speed drives (VSD) pump the screened influent from the wet well to the bioreactor. Each pump is capable of delivering 330 L/sec at a 7.6 metre head, with two pumps operating in duty mode and the third as standby. The existing wet well has provision for a fourth pump.

Down stream of the raw sewerage pump station but before the bioreactor is a magflow meter that logs the daily influent flow.

Top

Bioreactor

The bioreactor is configured as an anoxic-aerobic-anoxic-aerobic plug flow process and has volume of 20000m3, receiving screened raw sewage from the wet well.

The reactor basically consists of a mass of micro-organisms (activated sludge) that live and feed on the organic material in the influent which mainly consists of protein, carbohydrates and fine suspended organic matter. This mass of micro-organisms or activated sludge typically operates at a concentration of 2500 to 3000 g/m³ but at times maybe higher.

The concentration of activated sludge in the reactor is generally determined by how much waste activated sludge is removed and/or how well the activated sludge is settling within the secondary clarifier and the rate of return activated sludge to the bioreactor from the secondary clarifier. The concentration of solids within the bioreactor may be altered due to the wash out of solids after a storm event or solids being flushed through the sewerage network.

To break down the organic material micro-organisms require oxygen which is either supplied by aeration or is obtained from other sources within the biomass.

A dividing wall separating the inlet end of the reactor and the outlet end has four internal recycle pumps. The control philosophy of the internal recycle pumps is that the pumps will be required to operate proportionally to the influent flow. Under normal daily flows at most two internal recycle pumps would operate.

Return activate sludge (R.A.S) from the secondary clarifiers is introduced into the first anoxic zone, in the same area that raw sewage is introduced. A division wall forms a contact area at this point in the reactor, which allows the R.A.S to mix with the influent. R.A.S from number 2 secondary clarifier is introduce at the bottom of the reactor on one side of the division wall, R.A.S from number 1 secondary clarifier is introduce at the top of the reactor where it mixes with the biomass being recirculated by the internal recycle pumps.

The first anoxic zone has several submerged baffle walls creating 3 almost individual compartments, the last baffle wall is right up against the first aerobic zone. Within each anoxic zone is a submerged mixer to maintain suspension of the biomass without the introduction of air. The purpose of the anoxic zones is denitrification, the conversion of nitrites (NO₂ – N) and nitrates (NO₃ – N) to nitrogen gas. Certain micro-organisms obtain their oxygen for respiration from the nitrates and nitrites thus producing nitrogen gas.

There are three anoxic zones established within different areas of the bioreactor. Nitrates and nitrites are created by the use of aeration to oxidise ammonia (NH3 – N) called nitrification, which occurs in the aeration zones throughout the reactor. The reactor consists of two aerated areas; the first aerated area has three individually controlled zones. Each zone consists of five diffuser headers, and each zone has its own DO sensor and control valve.

The second aeration zone consists of four individually controlled zones, with a total of six diffuser headers per zone with their own DO sensor and control valve. Having individually controlled banks of diffusers allows flexibility in delivering air to where it’s required most.

Click on image to enlarge.

Top 

Waste Actived Sludge

Waste activated sludge (W.A.S) is the surplus amount of activated sludge, or put another way the micro-organisms surplus to requirements. The W.A.S is removed by diverting mixed liquor from the bioreactor over a weir and into the W.A.S pump station. From the pump station W.A.S is pumped to the sludge stabilisation pond. There are two W.A.S pumps, a duty/standby pump arrangement, each rated to pump 20 L/sec. The amount of activated sludge to be wasted is determined following a sludge solids inventory of the amount of total biomass within the reactor and clarifiers.

The daily amount of sludge to be wasted my not alter from day to day but as a process management tool a solids inventory should be done every day.

Top

Secondary Clarifiers

The function of the secondary clarifier is to provide a tank for activated sludge from the bioreactor to settle to the bottom. The activated sludge or micro-organisms as they settle leave a clear liquid, referred to as final effluent.

At this stage of the treatment process between 90 to 95 % of the suspended solids and BOD has been removed.

A centrally driven scraper arm moves slowly along the bottom of the clarifier dragging the settle solids to a central cone where the micro-organisms in the mixed liquor are returned to the bioreactor. This returned sludge is referred to as returned activated sludge (R.A.S). There are two R.A.S pumps per clarifier, always one pump as a standby.
All the return activated sludge from clarifier 2 is introduced back into the ditch at the point the raw sewage enters, return activated sludge from clarifier 1 is pumped back over the side of ditch. With the R.A.S return from number 1 clarifier there is flexibility within the pipe lay out that allows R.A.S to either be pumped directly in to the area where the recycle pumps recirculate MLSS back around the reactor or into the contact zone where raw sewage and R.A.S from number 2 clarifier enters. The other alternative is to split the flow 50/50 between the reactor recycle area and the contact area.
Surface scum on the secondary clarifiers is scraped to scum traps; there are two traps on each clarifier. Scum is discharged into a pump station located between the two clarifiers from which it is piped down to the waste sludge stabilising pond.

Top 

Process Building

The Process building houses the plant aeration blowers, R.A.S pumps and emergency standby generated. Part of this building is the motor control room.

Top

Motor Control Room

The motor control room is the home to all the switching gear and motor over loads for the R.A.S pumps, aeration blowers, and control valve actuators on the bioreactor, clarifier drive motors. There is a cabinet of batteries for UPS backup power for the Allen Bradley controllers for times of power outages.

Top 

Emergency Generator

A 260 kva emergency generator is stationed in the R.A.S room. The purpose of the standby generator is to supply electricity to essential items of plant at times of power outages.

Top

R.A.S Pumps

There are four R.A.S pumps, 2 per clarifier, always one duty and a standby; each is rated to pump 150 L/sec at a 4 meter head. As discussed under section 6.0 on clarifiers there are various points where R.A.S is returned back to the bioreactor.

Top 

Aeration Blowers

Air to the Bioreactor (Oxidation Ditch) is delivered using 2 ABS HST Turbocompressors and 2 HV Turbo centrifugal single stage blowers. The amount of air required is determined by the measuring dissolved oxygen levels in various zones within the bioreactor.

When there high demand for oxygen the ABS HST blowers operate while the smaller HV Turbo blowers operate at times of low air demand which is generally at night at times of low sewage flows. The volume of air produced a day will vary between 87000 to 170000m3. The objective is to maintain the dissolved oxygen levels at approximately 2 g/m3. 

Top 

Treated Effluent

The treated wastewater now referred to as effluent flows from the clarifier to an 8 hectare retention pond before passing through a 4 hectare man made wetland and then pumped out to sea through a 3 kilometre pipe line which extends 900 metres off shore at Omanu. This ocean discharge pipe handles the effluent from both Te Maunga and Chapel St wastewater treatment plants.

Top