refurbishment and modifications to improve primary treatment to
combat raw water quality deterioration and optimise particulate
by Stuart Parker & Shaun Murphy
Flocculation tank and DAF cell walls (May 2017) - Courtesy
WTW is located at the north end of the Rhondda Fawr Valley, at
the base of the Rhigos Mountain, in the village of Tynewydd at
an elevation of 1,000 feet. Water treatment works have operated
at the Tynywaun site since 1906 and the existing works was last
refurbished during the 1990s, with new filters to replace the
original pressure filters. The plant serves a population of
approximately 30,000 people. The Tynywaun Raw Water Reservoir
(approximately 24,200m3) is supplied to by three separate
sources; Llyn Faw/Nant Garreg, Mountain Spring and Ystrad Fernol
respectively. Over recent years the Nant Garreg source has
deteriorated and is subsequently isolated during periods of
heavy rainfall. The existing works require refurbishment and
modification to ensure enhanced treatment during periods of high
rainfall and turbidity and to allow utilisation of Nant Garreg
and increase the capacity from 10MLD to 12MLD.
Tynywaun WTW comprises
two stages of rapid gravity filtration. The first stage is
external and of concrete construction, the second stage is
internal and of stainless steel construction. Raw water is dosed
with aluminium sulphate for coagulation and gravitates to the
first stage rapid gravity filters.
The partially treated
water is then pumped to the second stage filter house, and dosed
prior to filtration with chlorine for disinfection and manganese
precipitation, lime for pH correction, and orthophosphoric acid
to inhibit plumbosolvency. The second stage filters remove
manganese from the water. Treated water is chlorinated and
pumped to the final water service reservoir. The final water is
then stored for a contact time within the 4ML capacity final
water service reservoir before entering the supply and
The design of Tynywaun
WTW is being carried out by Arcadis (previously Hyder). The
value of the design package is £800k and includes the design of
all aspects of the project.
Pilot trials: Jar tests
were conducted in summer 2015 to determine treatment issues and
offer potential process solutions. Process testing was conducted
on raw water samples from the Nant Garreg source reservoir to
compare coagulant dosing chemicals, the two of which were the
existing aluminium sulphate and an alternative ferric sulphate.
Raw water samples were taken during a period of significant
rainfall, which resulted in a high turbidity sample.
The established aluminium
sulphate chemical coagulant at Tynywaun was maintained, as the
process tests indicated that the aluminium sulphate produced a
better clarified water quality over the ferric sulphate
coagulant. It was also determined that the raw water pH
reduction anticipated by the aluminium sulphate dose should be
corrected with a new lime dosing point upstream, to ensure
alkalinity and aluminium precipitation for optimal clarification
and residual metal concentrations.
Primary flocculation and
DAF clarification was subsequently proposed following these
findings, strengthened by its proven ability in treating low
turbidity and moderate to high colour upland waters.
The proposed refurbishment and modifications required
construction of a new building to house the flocculation and DAF
plant together with the proposed lime, alum and poly storage and
dosing systems, along with the corresponding MCC room and a
laboratory for sample analysis.
Having increased the
energy requirements with the introduction of a new DAF system,
the environment and efficiencies were reviewed and improvement
was identified to utilise the site topography by removing the
existing interstage pumping between first and second stage RGFs
and replacing with a gravity main. The first stage RGF outlet
pipework was rerouted to suit the inclined site topography with
a gravity main configuration, thus eliminating the current need
First stage RGF
modifications also include improved filter flow control and
backwashing facilities with a new rinse-to-waste facility. The
rinse to waste is designed to divert initial flow of water
through a filter to waste after a backwash. This rinse water is
collected and pumped back to the raw water reservoir. A run to
waste facility has also been incorporated to enable the
treatment works to be flushed through, should the treated water
not meet quality standards.
orthophosphoric acid and chlorine storage dosing systems are
approaching the end of their asset life and are therefore being
replaced as part of the scheme.
Once the clarification stage had been agreed, the location and
layout of the works extension had to be designed to accommodate
the limited space available on the treatment works site. It was
considered advantageous to locate the DAF plant at an elevation
higher than that of the existing 1st stage filters to eliminate
the need for any interstage pumping.
The location onsite
selected for the new DAF building revealed some unexpected
issues during the ground investigations. Several buried tanks
were discovered beneath the site which had been part of the
earliest treatment works and consisted of slow sand filtration
and a water storage tank. Once discovered it became apparent
that the tanks were filled with rubble and covered with grass;
presenting unsuitable ground conditions to support the new DAF
building without ground improvements. Various groundworks
solutions were considered, before being narrowed down to two
suitable alternatives, ground stabilisation or piling.
Ground stabilisation was
potentially the most economical solution, but was ultimately
rejected as it would not have been possible to deliver the plant
and equipment to site due to the restricted access to the site.
Piling was finally chosen, and again the piling design was
limited to the size of piling rig that could be delivered to
site. These ground conditions also necessitated a review of the
proposed plant layout, to position the flocculation and DAF
tanks on the best load bearing area of the site.
Washwater from the works
(from filter washing and sampling) was designed to be collected
in lagoons and discharged to the sewer. A limit of 20m3/hr of
the site waste volume can be discharged to the sewer, presenting
a limited spare capacity remaining for the additional discharge
from the new DAF plant.
To minimise DAF sludge
volumes, a sludge scraper was incorporated into the DAF cell
design. This scraper pushes sludge over a beach rather than
hydraulic desludging; whereby a DAF stream is isolated, allowing
the tank level to rise until DAF sludge discharges over a weir.
Therefore, surface scraping produces less wastewater than
hydraulic desludging, hence maintaining the total wastewater
volumes within the permitted flowrate and eliminating the need
for an additional wastewater treatment process to reduce the
volume of wastewater discharge to sewer.
mat nearing completion
Courtesy of Skanska
Courtesy of Skanska
Site establishment at Tynywaun was in early November 2016, with
the first activities limited to excavation and piling. Many
issues presented challenges from a construction point of view.
Access to the site is gained through a small village, with pinch
points in the road, which restricted the size of vehicle access
Any level ground on the
topographically inclined site is accommodated with existing
works resulting in a scarcity of spare storage space. This
restriction meant that any excavated material had to be removed
from site in small lorries adding time and cost implications to
the construction activity. Deliveries associated with the
construction were time managed in line with the local school –
scheduled outside of start and finish times – to reduce risks
and promote safety.
The site access
restrictions have also limited alternative construction
techniques for implementation at Tynywaun. The previously
mentioned road pinch points ruled out the practicality of
factory assembled steel process tanks or precast panels;
therefore conventional concrete site construction methods were
required, which extended the construction programme.
impact has been at the forefront of the AMP6 improvement works
at Tynywaun WTW. Where process critical introductions have
resulted in increased energy consumption, the design team has
considerately reconfigured pipework to remove interstage pumping
and utilise the site topography for gravity operation. The
building for the new DAF plant was put through planning with
provision for solar panels to offset the energy usage. The
panels are outside of scope but planning was agreed with them
Initial phases of civil
construction began in the DAF plant in early 2017, with the
foundations piled, floor slab poured and works starting on the
DAF tank walls. Construction in the DAF plant is expected to
continue until late 2017. This will run in parallel with the
other site modifications, aiming for commissioning and phasing
the new plant into service by the end of March 2018.
and publishers would like to thank Stuart Parker and Shaun
Murphy of Arcadis Consulting (UK) for providing the above
article for publication. The authors thank Skanska for their
support in its preparation and Dŵr Cymru Welsh Water for
permission to publish this paper.