Background Over the past couple of years there has been an
increase in the consumption often drawn from the
Te Aroha Water Treatment Plant. Concurrently Inghams
Chicken Poultry Processing Plant, on the Hauraki
Plans, have increased their business significantly
and now require additional water. As a consequence
MPDC has undertaken to supply Inghams with potable
water. This has resulted in the Te Aroha Water
Treatment Plant being upgraded and capacity increased
from 4000 to 8000m3 /day.
The project was funded by MPDC with a significant
contribution from Inghams Chickens.
The upgrade had to be carried out while the plant
remained in operation, and in fact during the
latter part of the project increased production
was required.
The process selection for the upgrade involved;
Changing
the coagulant from Aluminium Sulphate to Poly
Aluminium
Chloride
doubling the number of filters and then automation
Retrofitting Tube Settlers
to the two existing
Clarifiers
Upgrading chemical storage
and dosing
Installation
of Turbidity Meters
The plant maintenance and
operations provider is Kaimai Valley Services
(KVS). Work was
allotted with civil and building components being
contracted to KVS with Filtration Technology carrying
out the mechanical and plant controls.
Change of Coagulant The raw water is supplied from up to five sources
that are blended in a balance tank that then discharges
to the treatment plant. Of the source three are
gravity fed, low turbidity, low alkalinity streams,
whilst the other two are pumped, one of which
is the Waihau River. Gravity sources are the preferred
supplies. With such varied raw water sources,
we determined that Poly Aluminium Chloride would
produce a faster forming floc and have less dependence
on raw water alkalinity. A further advantage of
the Poly Aluminium Chloride is its greater tolerance
to changes in raw water. By changing the coagulant
the plant capacity increased.
Filters/Automation The existing filters were very much in an ‘as
built’ condition, largely unchanged since
construction in 1967. Their control had been modified
however, with discharge flow controlling the filtered
water valve. The filter inlet channel water depth
controlled the plant inlet flow.
To relieve production pressure, two epoxy coated
steel filters constructed and installed adjacent
to the existing plant. Although round these filters
have the same surface area and operation depth
as the original filters.
A new control system was installed on all the
filters with ultrasound level sensors adjusting
the filter outlet valves to maintain a constant
filter water level. Backwashes are initiated when
the level begins to rise. Raw Water Mag Flow Meter
now has direct control of the raw water inlet
valve. There is a secondary pressure sensor in
the filter inlet channel as a failsafe to plant
flooding.
Retrofitting Clarifiers
with Tube Settlers By installing Tube Settlers into a Clarifier the
rise rate can be doubled from typically 2m3 /m
2 /hr to 4.5m3 /m 2 /hr. We have thereby been
able to double the clarifier’s capacity without
needing to add civil structures. This has shown
good cost saving on alternative options. Installed
the Tube Settlers have required some changes in
the way the plant is operated. There is no longer
a clarified water blanket for the operator to
visually inspect and desludging cones are no longer
used. Jar test, sludge index test and clarified
water turbidity have become chief operating tools.
Upgrading of Chemical
Storage and Dosing With the increased water production, the chemical
storage and dosing systems were too small. In
order to make plant operation easier, larger storage
vessels for Chlorine and LiquiPAC have been installed.
Dosing Pumps have been re-sized.
Installation of Turbidity
Meters To meet requirements of both DWSNZ and contract
supply, online turbidity monitoring has been added
to all filter output.
