Paraparaumu Sewage Treatment Plant - Overview · Paraparaumu Sewage Treatment Plant - Overview 1.1...

Page: 1-2-1 Rev: 4

Paraparaumu Sewage Treatment Plant - Overview

1.1 BACKGROUND Paraparaumu WWTP was commissioned in 1980 with a design population capacity of 12,500. The original extended aeration process was upgraded in 1994 to incorporate biological nutrient removal and UV disinfection for a 25,000 equivalent population.

The 2001 upgrade increased the plant capacity to 39,500 equivalent population and improved the effluent in terms of faecal coliforms, dissolved reactive phosphorus (DRP) and nitrate nitrogen.

The principal changes of the 2001 upgrade include:

• Upgraded facilities at Rauparaha Street and Waikanae Pump Stations;

• A new inlet works consisting two rotary screens and a grit removal unit. The existing influent channel and step screen are retained as a standby and bypass;

• two new anaerobic zones and two new primary anoxic zones;

• the existing anaerobic and anoxic zones are upgraded to secondary anoxic tanks;

• two new aeration zones;

• the existing surface aerators are replaced by diffused air system;

• one new clarifier;

• two new RAS pump stations;

• a new UV Disinfection plant;

• upgraded existing DAF.

The required standards are:

Parameter Consent Contract

Effluent BOD Effluent g/m³ 1 15 = geometric mean BOD Effluent g/m³ 1 25 = <=1 per month to exceed Amm-N Effluent g/m³ 3 3.6 = <=3 of 36 samples to exceed TSS Effluent g/m³ 1 15 = geometric mean TSS Effluent g/m³ 1 25 = <=1 per month to exceed Nitrate-N Effluent g/m³ 3 10 geometric mean Nitrate-N Effluent g/m³ 3 30 = no sample to exceed Total P Effluent g/m³ ReportDRP Effluent g/m³ 3 Report 0.5 geometric mean FC Effluent CFU/100ml 2 200 50 geometric mean DO Effluent ppm 4 5.0 = <=3 of 36 samples to be below pH Effluent low 2 6.0 = no sample to exceed pH Effluent high 2 9.0 = no sample to exceed Arsenic (III) g/m³ 5 0.01 = no sample to exceed Copper g/m³ 5 0.01 = no sample to exceed Chromium (VI) g/m³ 5 0.01 = no sample to exceed Cadmium g/m³ 5 0.004 = no sample to exceed Nickel g/m³ 5 0.1 = no sample to exceed Mercury g/m³ 5 0.0002 = no sample to exceed Lead g/m³ 5 0.01 = no sample to exceed Zinc g/m³ 5 0.1 = no sample to exceed

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Solids Thickened WAS %Dry solids 3.5% ± 0.5

Notes

1. Based on no fewer than 12 flow proportional samples per month

2. Based on no fewer than 12 representative samples per month

3. Based on 36 consecutive and representative samples collected at rates of no fewer than 6 samples per month and 1 per week

4. Based on 36 consecutive and representative samples collected at rates of no fewer than 12 samples per month

5. Based on no fewer than 1 sample per month

1.2 OVERVIEW OF THE FACILITIES The flow path of upgraded WWTP is shown in Figure 1.

The inflows to the WWTP come from:

Wastewater from Waikanae – this is collected at Waikanae Pump Station and is pumped via Rauparaha Street Pump Station, together with flows from the local sewer network.

Wastewater from Paraparaumu.

Leachate from the Otaihanka Landfill.

Septage delivered to the site by road tankers.

At the WWTP the influent first passes through preliminary treatment, consisting of fine screens and grit removal, before biological treatment. The biological treatment is an activated sludge system in the Modified Johannesburg configuration. After settlement the final effluent is disinfected by UV light before being discharged into Mazengarb Waterway.

In wet weather the flows to the plant are stored in the activated sludge plant and in severe or prolonged wet weather offline storage is provided in an open storage basin.

Excess biomass generated by the plant is thickened in a Dissolved Air Floatation (DAF) plant before being vitrified in the Lemar Sludge Vitrification Plant.

PARAPARAUMU WWTP OPERATION & MAINTENANCE MANUAL

Page: 1 - 4

InletWorks

AerationZones

DisinfectionClarifiers2° AnoxicZones

AnaerobicZones

SludgeThickening

StormStorage

MazengarbWaterway

Vitrified sludge

1° AnoxicZones

WAS

RAS

A-Recycle

R-Recycle

SeptageSump

LEMAR Plant

RauparahaSt PS

WaikanaePS

Paraparaumu

Leachate

Waikanae

Waikanae Black Creek Drain

Septage

Main FlowIntermittent Gravity FlowIntermittent Pumped Flow

Sludge TreatmentReturn Sludge Liquor

Figure 1 - Flow Schematic of Paraparaumu WWTP

Rev: 4

PARAPARAUMU WWTP

Section 2 - Description of Systems

2.1 PROCESS OVERVIEW The upgrade of the existing Paraparaumu WWTP has been designed in such a way that maximum use has been made of existing structures without sacrificing flexibility and ease of operation.

The incoming wastewater flows through a new inlet works consisting of two rotary screens and a vortex degritter. A bypass for the mechanical screens has been provided utilising the existing step screen channel. A stormwater bypass has also been provided after the screens to divert excessive and sustained peak flows to a storm pond.

After the vortex degritter the wastewater discharges to the biological treatment plant. The biological treatment consists of four zones where the conditions are controlled to promote the required biological activity; this arrangement is called the ‘Modified Johannesburg Configuration’.

SECONDARYCLARIFIERSPRIMARY

ANOXICZONES

ANAEROBICZONES

SECONDARYANOXICZONES

AEROBICZONES

Controlled DOOxidation of BODBOD + O2 H2O + CO2Ammonia to NitrateNH4 NO3 + H2OUptake of Phosphorus

A-RecycleHigh flow nitrate recycle

No DONitrate to gasNO3 O + N2

No DONitrate to gasNO3 O + N2

RASML (Biomass) separation from the final effluent

No DONo nitrateRelease and conversion of P

RawWastewater

R-RecycleRecycle of ML & BOD

WASTo Sludge Thickener

The four zones are:

Primary Anoxic – In the primary anoxic zones (DNA and DNB) the return activated sludge (RAS) is mixed with the R-Recycle flow. The purpose of these zones is to remove all the nitrates in the RAS before being mixed with the raw wastewater. The nitrates are removed by denitrification; bacteria (in the RAS and the R-Recycle) are presented with food (in the R-Recycle) in the absence of dissolved oxygen. When there is no easily available oxygen the respiring bacteria will take chemically bound oxygen, in this case from the nitrates:

Overview - PSTP Configuration & Process:pdk:3/05/2004@10:06 a.m. Page 2 of 19

PARAPARAUMU WWTP

↑+⇒ 23 NONO

Anaerobic – The anaerobic zones (ANA and ANB) are where any form of oxygen is excluded. The raw wastewater is mixed with the effluent from the primary anoxic zones. In these conditions (sometimes called ‘famine’ conditions, with no available oxygen; dissolved or chemically bound) some micro-organisms release phosphorus as ortho-phosphate into the liquid. The energy released in this action is used to store the short chain fatty acids for future use in the anoxic and aerobic zones. The bacteria present also convert suspended solids and dissolved slowly degrading organic matter into readily available BOD.

Secondary Anoxic – The purpose of the secondary anoxic zones (DN1 to DN5) is the same as the primary anoxic zones. The flow from the anaerobic zones is mixed with the A-Recycle flow. The A-Recycle flow is relatively high (around 8-9 times the plant inflow) and recycles a nitrate rich liquor, from the aerobic zones, which is denitrified when it is mixed with the food from the anaerobic zones in the absence of dissolved oxygen.

RAS PS#2

RAS PS#1

A-Recycle

SecondaryAnoxicZones

PrimaryAnoxicZones

ANAANB

DN4DN3

DN2 DN1

DN5

AE3

AE5AE2

AE1 AE6

AE4

DNA DNB

AnaerobicZones

AerobicZones

R-Recycle

Aerobic – The key biological processes occurring in the aerobic zones:


PARAPARAUMU WWTP

Carbonaceous Oxidation – the conversion of organic carbon compounds to water and carbon dioxide

↑+⇒+ 22xxx COOHOOHC

Nitrification – the oxidation of ammonical compounds to nitrites and then to nitrates

OH NOO OHNOONH 23224 +⇒++⇒+

Phosphorus Uptake – When entering the aerobic zones the microorganisms that stored short chain fatty acids in the anaerobic zones utilise these acids as food for growth and to replenish their stores of phosphorus. They compensate for the ‘famine’ conditions in the anaerobic zones by taking up excessive phosphorus. This process is often called the ‘luxuriant uptake of phosphorus’. These micro-organisms retain the phosphorus and it is removed from the liquid stream when the bacteria are wasted in the WAS.

The mixed liquor from the biological treatment plant discharges into a distribution chamber where the flow is split via overflow weirs to three clarifiers. The settled sludge from the clarifiers is recycled to the primary anoxic tanks (DNA & DNB).

The overflow from the clarifiers discharges to a UV disinfection channel and the disinfected effluent gravitates to the Mazengarb Drain via a new pipeline beneath the storm storage pond.

Waste activated sludge abstracted from the aerobic zones discharges to a Dissolved Air Flotation (DAF) thickener. The thickened sludge is pumped to the existing centrifuge and then fed to the Lemar Sludge Vitrification Plant. The subnatant from the DAF unit and the centrate from the centrifuge are returned to biological treatment plant.


PARAPARAUMU WWTP 2.2 CONTROL SYSTEM

Figure 2 - Schematic of the Control System

The WWTP equipment is monitored and controlled through one main PLC (Modicom Quantum). Several individual process units and the Pump Stations have their own local control systems with the UV Disinfection plant being controlled having its own PLC.

The user interface to the PLC is the Development PC in the WWTP Control Room running the Intouch Graphical User Interface (GUI) application. This PC forms part of the plant SCADA system and is used for changing set-points, processing alarms and accessing current and historical plant data. This PC also has access to local control systems at Waikanae and Rauparaha Pump Stations and can be used to monitor and control them in the same way as the rest of the plant.

Through the Terminal Server up to 5 other PCs can access the plant control system and be used to view plant operating data, alarms and to change set-points. These other PCs can be over the local network or remotely over a modem.

The system is shown in Figure 2.

2.3 WAIKANAE PUMP STATION Waikanae Pump Station receives wastewater from the Waikanae local sewerage system. The Pump Station has two wet wells; one containing two pumps transferring dry weather flows up to 80 l/s to Rauparaha Street Pump


PARAPARAUMU WWTP

Station and a storm wet well containing two storm pumps. In the event of the inflow exceeding 80l/s the sump will overflow into the Storm Wet Well. The two Storm Pumps pump storm water from the Storm Wet Well to Black Creek Drain via a step screen and flow meter.

2.4 RAUPARAHA ST PUMP STATION Rauparaha St Pump Station receives flows from Waikanae Pump Station and from the local gravity sewer. Two pumps, operating as duty/standby, transfer wastewater to Paraparaumu Wastewater Treatment Plant via a flow meter. The maximum flow is 137 l/s.

The sump is covered and escaping air is passed through an activated carbon odour control unit.

2.5 SEPTAGE IMPORTS The septage receival facility is designed to receive septage wastes from tankers and permit its discharge to the influent channel.

The septage receival facility comprises a hard stand concrete apron and a camlock upstand for discharge of the septage. The upstand discharges into the Septage Receival Tank, which has a capacity of 25m³.

A manually initiated actuated valve transfers the contents of the Septage Receival Tank into the Inlet Channel at a controlled flow rate.

The Tank is covered with the headspace being extracted to the odour control system (2.9). The headspace atmosphere is also monitored for Lower Explosive Limit (LEL - hydrocarbon gases) and hydrogen sulphide gas with alarms for potentially explosive or toxic atmospheres. A high level LEL alarm will also sound the audible alarm and close the actuated discharge to prevent contamination of the influent wastewater.

2.6 STORM STORAGE Included in the Inlet Works structure is a fixed weir set to divert wet weather flows in excess of the capacity of the biological treatment to the Storm Storage Basin. This forms a part of the Flow Control System, described in sub-section 2.10. The Storm Storage Basin has a nominal capacity of 25,000m³.

When the inflows to the WWTP subside the wastewater diverted to the Storm Storage Basin will be automatically returned to the Inlet Works, upstream of the screens, for full treatment. The Storm Return Pump Station consists of two submersible pumps operating as duty/assist. The flow capacity of one pump is 65l/s and two pumps 110l/s.

If, in the event of prolonged wet weather, the Storm Storage Basin fills then the flows will overflow direct to Mazengarb Drain.

Details of the operation of the Storm Storage facility can be found in sub-section Error! Reference source not found..


PARAPARAUMU WWTP 2.7 SCREENS

VortexDegritterParaparaumu

Leachate

SeptageSumpSeptage

Flow meter

FlowmeterFlow meter

Waikanae

Waikanae

RotaryScreen A

RotaryScreen B

StandbyStep Screen

RauparahaSt PS

AnaerobicZones

Black Creek Drain

WaikanaePS

StormStorage

Figure 3 – Schematic of the Inlet Works

The mechanical screening system is provided to remove gross solids, plastics, paper, rags and other fibrous material from the wastewater inflow and septage to protect downstream mechanical equipment and minimise blockages of pumps and pipes. Facilities are provided to minimise the amount of organic material entrained with the screenings and to dewater the screenings to minimise the volume and weight of screenings requiring disposal.

The screening system comprises of two duty rotary fine screens and a standby step screen.

The rotary fine screens are the rotary drum type with 5mm diameter holes. The rotary fine screen system is designed to have a maximum throughput of 650 l/s, with both screens in operation.

Each screen is fitted with an integral conveyor/compactor and an automatic washing system. Both screens discharge into a horizontal screw conveyor that transfers the screenings to the skip. Plastic bags are attached to the end of the conveyor, to contain any odour, and placed in a skip.

The existing 6mm aperture step screen is retained as a standby unit. In the event of a malfunction of the rotary screen system and/or high inflows, flows will overflow into the Step Screen Channel. A hydraulic ram conveyor transfers the screenings from the step screen to a bagging arrangement.

Emergency relief is provided with high level overflow around the side of the Step Screen and to an emergency bypass channel.


PARAPARAUMU WWTP 2.8 GRIT REMOVAL

The grit removal system is provided to remove grit from the flow to reduce erosion and damage to downstream equipment. Facilities are provided to minimise the amount of organic material and water entrained with the grit to minimise the volume and weight of grit requiring disposal.

The grit removal system comprises of a vortex flow pattern chamber, a removal pump and a grit classifier. The Vortex Degritter is designed to maintain constant flow velocity by using the scimitar type paddles rotating continuously in the same direction as the wastewater flow to create a condition for grit separation, settling and organics upflow. The grit removal system has a maximum flow capacity of 650 l/s.

Prior to being removed from the Vortex Degritter by the grit pump, the settled grit is washed by the injection of washdown water. The grit is pumped to the classifier where it is settled, the supernatant water being returned to the inlet channel by gravity. The grit is transferred up the classifier and into a skip by a screw conveyor. While in the screw conveyor it is further washed by washdown water spray nozzles.

2.9 ODOUR CONTROL The Inlet Structure and the Septage Receival Sump are covered and the headspace atmosphere is extracted by the Odour Control Blower. The odorous air is diffused in to Aeration Zone 6 (AE6) for biological treatment.

2.10 FLOW BALANCING The flows through the WWTP are attenuated by storage in three steps:

The flows through the Mixed Liquor Distribution Chamber are limited to 290l/s. As the inflows to the WWTP rise above this the water level in the aeration zones will rise.

At a preset level (measured in DN3) the actuated penstock between zones DN2 and DN3 is brought into operation and modulates to maintain this level. By restricting the flows, the water level in the zones upstream of this penstock (ANA to DN2) rise further.

As the inflows increase further level rises and overflows the weir to the Storm Storage Basin

Also, as a standby to these operations:

If there is any failure of the above system the inlet flow control penstock, upstream of the Vortex Degritter will be brought into operation and modulate to limit the inflows to 290 l/s, using the influent flow meter to measure the flow.


PARAPARAUMU WWTP

In line storage 700 m3

In line storage 130 m3

To Clarifie

Off line storage 25,000 m3

Zones ANA - DN2 Zones DN3 - AE6

Weir to Storage

ML Distribution Chamber

FIC

Figure 4 -Flow Balancing Schematic


PARAPARAUMU WWTP

A-Recycle

ANAANB

DN4DN3

DN2 DN1

DN5

AE3

AE5AE2

AE1 AE6

AE4

DNA DNB

AerobicZones

R-Recycle

Mixed Liquor Distribution Chamber

Inlet Flow Control Penstock

Secondary AnoxicFlow Control Penstock Overflow to Storm Storage Pond

Pumped return from Storm Storage Pond

Figure 5 - The Flow Balancing System

2.11 PRIMARY ANOXIC ZONES A portion of the flow exiting the anaerobic zones is pumped by the R-Recycle pumps into the two primary anoxic zones (DNA and DNB) where it is mixes with RAS from RAS pump stations 1 & 2. The purpose of the primary anoxic zones is to allow the biomass in the RAS to denitrify the RAS that is to strip the nitrates (NO3

-) of oxygen. This is to prevent any leakage of nitrate into the anaerobic zones and cause the limitation of phosphorus removal. This results in the release of nitrogen gas (N2).

The anaerobic zones are provided with two submersible mixers (MX203 and MX204), one in each zone, to keep the contents of the zones in suspension. The two zones have a combined capacity of 446 m³; the individual tank volumes are shown in Table 2-1.


PARAPARAUMU WWTP

The outflow from the second primary anoxic zone (DNB) flows into the first anaerobic zone (ANA) where it mixes with the wastewater influent.

2.12 ANAEROBIC ZONES Screened raw sewage flows from the Inlet Works to two anaerobic zones (ANA and ANB), via the inlet flow meter, where is mixed with flow from the primary anoxic zones. In these conditions (sometimes called ‘famine’ conditions, with no available oxygen; dissolved or chemically bound) some micro-organisms release phosphorus as ortho-phosphate into the liquid. The bacteria present also convert suspended solids and dissolved slowly degrading organic matter into readily available BOD.

The anaerobic section comprises of two zones in series. The anaerobic zones are provided with two submersible mixers (MX201 and MX202), one in each zone, to keep the contents of the zones in suspension. The two zones have a combined capacity of 1,228 m³; the individual tank volumes are shown in Table 2-1.

The R-Recycle Pumps pump part of the outflow from the second anaerobic zone (ANB) into the first primary anoxic zone (DNA), to provide a nitrate free supply of organisms to the Anaerobic Zone. The remainder of the outflow flows by gravity into the first secondary anoxic zone (DN1).

The flow pumped by the R-Recycle Pumps is maintained at a set proportion of the WWTP inflow. The maximum flow capacity of the R-Recycle Pumps is 145 l/s.

2.13 SECONDARY ANOXIC ZONES The Secondary Anoxic section consists of 5 zones in series (DN1 to DN5). The flow from the Anaerobic Zones enters the first zone along with the A-Recycle flows from the last aerobic zone. The purpose of the Secondary Anoxic Zones is the removal of nitrates from the A-Recycle flows. As in the Primary Anoxic Zones, in the absence of dissolved oxygen the biomass will strip oxygen from the nitrate radical (NO3

-) to release nitrogen gas (N2).

The Secondary Anoxic Zones are provided with five submersible mixers (MX205 to MX 209), one in each zone, to keep the contents of the zones in suspension. The combined capacity of the zones is 3,394 m³; the individual tank volumes are shown in Table 2-1.

The outflow from the last secondary anoxic zone (DN5) flows into the inlet of the first Aerobic Zone (AE1).

At times of high inflows to the WWTP the flow between the second and the third secondary anoxic zones is controlled by the flow control penstock (MOV 0237)

2.14 AEROBIC ZONES The Aeration section is the next step in the treatment process and is provided with a Fine Bubble Diffused Aeration (FBDA) system to supply air:


PARAPARAUMU WWTP

for the oxidation of carbonaceous material

for the conversion of ammonia to nitrates

to provide conditions for the luxuriant uptake of phosphorus

to keep the contents of the zones in suspension.

The aerobic section consists of six zones in series (AE1 to AE 6). The combined capacity of the zones is 5,994 m³; the individual tank volumes are shown in Table 2-1.

Table 2-1 Zone Dimensions

Tank BW Depth m

TW Depth m

Width m

Length m

BW Volume m³

TW Volume m³

DNA 3.95 4.94 4.90 9.20 179 223

DNB 3.95 4.94 4.90 9.20 179 223

Total Primary Anoxic Volume 358 446

ANA 3.34 4.94 9.20 13.50 415 614

ANB 3.34 4.94 9.20 13.50 415 614

Total Anaerobic Volume 830 1,228

DN1 3.81 5.37 11.00 17.00 712 1,004

DN2 3.81 5.30 11.00 17.00 701 991

DN3 4.41 4.70 7.35 13.70 444 473

DN4 4.39 4.63 7.35 13.70 442 466

DN5 4.36 4.56 7.35 13.70 439 459

Total Secondary Anoxic Volume 2,738 3,394

AE1 4.74 4.97 13.30 13.30 838 879

AE2 4.71 4.94 13.30 13.30 834 874

AE3 4.68 4.91 13.30 19.00 1,184 1,242

AE4 4.66 4.89 13.30 19.00 1,177 1,235

AE5 4.63 4.86 13.30 13.30 819 859

AE6 4.60 4.83 13.30 13.30 814 854

Total Aerobic Volume 5,665 5,944

Total Volume 9,591 11,011

Three positive displacement blowers, and a grid of fine bubble diffusers in each of the zones, supply air to the Aerobic Zones. Dissolved oxygen (DO) probes in each of the zones control the level of aeration by adjusting airflow control valves and the speed of the blowers.


PARAPARAUMU WWTP

The aeration system is the most energy intensive part of the treatment process and care is required to optimise the aeration to maintain efficient operation while maintaining treatment performance.

Part of the outflow from the last aerobic zone (AE6) is pumped, by the A-Recycle Pumps, into the first secondary anoxic zone (DN1) with the remainder flowing by gravity into the clarifiers via the Mixed Liquor Distribution Chamber.

Scum is manually removed from the aeration zones by a weir penstock. When lowered scum from the outlet channel between AE5 and AE6 flows by gravity to the scum pump station.

2.15 WAS CONTROL Sludge wasting is carried out to prevent excessive accumulation of solids in the biological treatment system and to prevent over ageing of the biomass. An excessive accumulation of solids within the biological treatment system overloads the secondary clarifiers with solids. Over ageing of the biomass can result in:

• poor flocculation and carry over of fine suspended solids in the secondary clarifier effluent

• excessive use of power to keep the biomass aerated

Over wasting can result in the reduction of the biomass to a level where treatment performance is adversely affected.

Sludge is wasted from the end of the aerobic zone as Mixed Liquor Suspended Solids. This method of wasting permits the use of hydraulic sludge age control. This automatic method of sludge age control eliminates the need to estimate sludge production or calculate a target Mixed Liquor Suspended Solids concentration.

To achieve hydraulic sludge age control, a fixed volume of sludge is wasted every day from the aerobic zone. Calculated as shown below:


PARAPARAUMU WWTP

Age Sludge Required9,770

/day)(m Volume WAS

used is zones those of volume the1.5 DNB) & (DNA zones anoxic primary the inhigher is MLSS the because However,

(days) Age Sludge Required)(m Volume TreatmentBiological

/day)(m Volume WAS

or /day)(m flow WAS

)(m Volume TreatmentBiological(days) Age Sludge

MLSS ionconcentrat solids WAS the then wasted isliquor mixed If

/day)(m flow WAS(mg/l) Solids WAS)(m Volume TreatmentBiological(mg/l) MLSS

/day)(m flow WAS(mg/l) Solids WAS(kg) Biomass Total

Plant the in biomass the of time Residence (days) Age Sludge

3

33

3

3

3

3

3

=

×

=

=

=

××

=

×=

=

The sludge age required to give the required performance has been determined as 14 days. This is based on computer simulation and will require confirming by the actual performance of the plant.

An actuated valve is provided to bleed mixed liquor from the A-Recycle Discharge Channel and the Waste Activated Sludge (WAS) flows by gravity through a flow meter to the sludge thickening plant. The flow of WAS is controlled by a feedback control loop using the WAS flow meter.

2.16 A‐RECYCLE PUMPING The five A-Recycle Pumps are designed to return the majority of the nitrate formed in the Aerobic Zones to the Secondary Anoxic Zones for denitrification. The A-Recycle Pumps deliver flow from the last aerobic zone (AE6) to the first secondary anoxic zone (DN1). At the end of the aerobic zones, almost all of the influent ammonia-nitrogen and organic nitrogen has been converted to nitrate. The return of this nitrate to the Secondary Anoxic Zone exposes the flow to the anoxic conditions and biodegradable substrate required for effective denitrification.

The maximum flow capacity of the A-Recycle Pumps is 955 l/s; the individual capacities are shown in Table 2-2


PARAPARAUMU WWTP

Table 2-2 A-Recycle Pump Capacities

Pump Capacity l/s

A-Recycle Pump A Variable Speed 110

A-Recycle Pump B Fixed Speed 110

A-Recycle Pump C Fixed Speed 110

A-Recycle Pump D Fixed Speed 315

A-Recycle Pump E Variable Speed 315

Total 955

2.17 ALUM DOSING The biological treatment processes at the WWTP will normally readily achieve the required Dissolved Reactive Phosphorus (DRP) level in the final effluent. However, changes in the biomass environment, for example temperature change or a toxic substance or stray biological culture in the inflow to the WWTP, may cause an upset. Alum dosing is provided as a back up to the biological phosphorus removal process.

When required the alum is dosed to the first Aerobic Zone (AE1). The removal of phosphorus by alum requires an accumulation of aluminium hydroxide in the mixed liquor suspended solids. As this accumulation occurs the extent of phosphorus removal will increase. Once the target effluent phosphorus concentration has been achieved, the alum dose can be decreased to maintain the target level. As the biological phosphorus removal system recovers, the alum dose can be further decreased and ultimately stopped.

The Alum Dosing facility consists of two storage tanks and a pumped dosing system. The capacity of the tanks is 10m³ each. For safety the tanks are located in a bunded area and a safety shower and eyewash is provided.

2.18 CLARIFIERS Mixed liquor is split three ways, in the Mixed Liquor Distribution Chamber, to flow to the central chamber of each of the three clarifiers. The weir lengths in the chamber apportion the flow.

In the clarifiers the mixed liquor separates into the supernatant, which overflows the peripheral weirs and a subnatant underflow, the RAS. The RAS is continuously withdrawn from the three clarifiers by different methods:

Clarifier 1 – The RAS is collected by siphon pipes on the rotating half-bridge and transferred to a chamber attached to the central column. The RAS then flows out from this chamber through the base of the clarifier to the RAS Pumps.


PARAPARAUMU WWTP

Clarifiers 2 & 3 – The RAS is scraped along the base of the clarifier, by scraper blades attached to the rotating half-bridge, towards a central hopper. The RAS then flows from this hopper to the RAS Pumps.

The peripheral speed of each half-bridge is manually set.

High-pressure water jets clean the weirs of clarifiers 1 & 2, the weir of clarifier 3 is cleaned by brushes.

Some solids will not settle within the clarifiers. This material remains as scum on the surface of the secondary clarifiers. The scum is prevented from discharging with the treated water by scum baffles around the circumference of each clarifier. Scum is removed from the clarifier surfaces by the scum removal system and pumped to the sludge thickener by the Scum Pump Station.

Table 2-3 Clarifier Dimensions

Side Wall Depth m

Int. Diameter m

Surface Area m²

Clarifier 1 2.7 23 415



2.19 RAS PUMPING The Return Activated Sludge (RAS) Pumping returns settled solids from the secondary clarifiers to the biological treatment zones. Effective RAS pumping prevents solids accumulation in the secondary clarifiers, and maintains a high concentration of organisms within the biological treatment zones to carry out the biological treatment.

The underflow from clarifiers 1 & 2 flows to RAS Pump Station 1 and the underflow from clarifier 3 flows to RAS Pump Station 2. Both pump stations transfer RAS to Primary Anoxic Basin AN1.

The RAS pumps are all variable speed pumps and the overall RAS flow rate is controlled to either, maintain a constant flow, or a set proportion of the WWTP inflow. The RAS pumps of each pump station operate as duty/assist. The flow capacities are – RAS Pump Station 1 102 l/s, RAS Pump Station 2 150 l/s.

The manual valve on the underflow of clarifier 2 adjusts the split between clarifiers 1 & 2. The split of total RAS flow pumped from each of the two pump stations is maintained at a set level to match the influent flow split to each of the clarifiers.


PARAPARAUMU WWTP 2.20 UV DISINFECTION

The overflow from the three clarifiers combines and is disinfected by exposure to ultraviolet (UV) light. UV disinfection is provided in a single open channel fitted with two banks of UV lamps, operating as duty/assist. The level of liquid in over the lamps is controlled by a weir penstock immediately downstream of the channel. The UV dose is controlled by the flow rate; measured by the flume downstream of the channel and the intensity monitors located in the two banks of lamps.

An automatic wiping system cleans the UV lamps.

2.21 WASHDOWN WATER Final effluent is taken downstream of the UV channel and pressurised for use on the WWTP by the Washdown Water Pump. The washdown water is used for:

Cleaning of the rotary screens and washing the screenings

Cleaning of the grit in the Vortex Degritter and the Grit Classifier

General washing of tanks and equipment using a number of taps located around the WWTP site

An in-line filter is provided to prevent blockages on the Washdown Water system.

2.22 SLUDGE THICKENING Waste Activated Sludge (WAS) is thickened by a Dissolved Air Floatation (DAF) Thickener. The DAF Recycle Pump circulates the contents of the DAF thickener through an air injection sparger and a saturation vessel to provide a liquid saturated with air. The WAS enters this recycle loop just before the flow is fed into the thickener tank via a central feedwell. On entry into the thickening tank air bubbles are released and cause the sludge particles to rise to the surface of the tank. The thickened sludge is scraped off the surface into a hopper by a rotating scraper. The thickened sludge flows from the hopper into a wet well from where it is pumped to the Lemar Sludge Vitrification Plant. The rotating scraper is powered via a variable speed drive, which is adjusted to achieve a maximum float concentration.

The air for the DAF is taken from the plant air supply. The airflow and the pressure in the recycle loop are controlled by a local control panel. The DAF Thickener is designed to operate continuously.

Table 2-4 DAF Tank Dimensions

Side Wall Depth m

Int. Diameter m

Surface Area m²

DAF Tank 8 50


Paraparaumu Sewage Treatment Plant - Overview · Paraparaumu Sewage Treatment Plant - Overview 1.1...

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