DRAINAGE, PLUMBING & WATER SUPPLY
JULY 8 | BUILDING PRODUCTS
by flotation. Dense non-aqueous phase liquids
(DNAPLs) can be separated via gravity using
lamella plates. The equipment deployed will vary
according to the oil encountered and again is
mobile and can be bought or hired.
Concrete Wash Water
Concrete wash water has the potential to cause a lot
of harm. This is because the pH of concrete wash
water is incredibly high – typically 12 to 13, which is
the equivalent of oven cleaner.
This highly polluting water needs to have its
suspended solids removed and its pH reduced to
an acceptable value. The solids removal is relatively
straight forward, however choosing the best pH
adjustment method is less clear-cut.
Dilution isn’t practical or cost effective as the
pH scale is logarithmic, whilst mineral acids are
dangerous to handle and can easily overshoot the
target resulting in acidic water. Citric acid is also easy
to overdose, and it increases the water’s biochemical
oxygen demand above acceptable limits.
Carbon dioxide is by far the best neutralising
agent. It’s virtually impossible to acidify water using
CO2, it has no hazardous by-products, is easy to
store and is the most cost-effective.
On-site treatment in action
But is on-site water treatment viable? To answer this,
let’s consider a few of examples.
We created and installed a processing plant
sometimes twice per day, driving waste to an off-site
disposal or recycling facility and returning to the site.
With a London sweeper costing circa £50 per hour,
this was an expensive amount of unproductive time.
Furthermore, the site’s waste disposal charges were
excessive, particularly during wet weather when the
sweeper vehicle was filled with dirty rainwater which
could not simply be released to the sewer.
Drawing on expert advice, the company
decided to manage the water treatment on-site,
using a hook-lift sweeper waste dewatering
system. This has made things much more efficient.
Indeed, Alandale has been able to reduce the
number of full time sweeper vehicles from two
to one. After dewatering, the separated-out water
is cleaned and has its pH adjusted using a pHD
unit. It is then safely discharged to foul sewer
under consent from Thames Water. With the water
removed, the dewatered solid waste only needs
to be sent for recycling once every two weeks,
reducing disposal costs by more than 50%.
The Environment Agency’s RPS is the strongest
signal that construction companies need to plan for
wastewater. I’d urge those involved in that planning
not to simply pick up the phone to a tankering
company but to consider on-site water treatment.
Those that do will reduce costs, ease their site’s
logistics and improve its environmental footprint.
in just seven days to remove dissolved iron from
groundwater as part of the £1 billion project
commissioned by National Grid and Scottish Power
Transmission to build the Western Link, bringing
renewable energy from Scotland to England and
Wales. Enabling the contractors to safely treat the
water on-site saved a lot of money. Tankering the
groundwater off-site for treatment would have
required an articulated tanker (50m3) every hour,
24 hours a day, at huge costs both financially (cost
£150-£250 per cubic meter) and to the environment,
in terms of carbon footprint.
During the construction of new buildings at
Alder Hey Children’s Hospital in Liverpool, massive
amounts of concrete were poured. The construction
crew initially tried using skips to deal with the
highly polluting washwater but when that proved
unsuccessful, they turned to a pH adjustment
and water recycling unit suitable for dealing with
washwater. The team used this for six months,
treating the water in-situ so that it was safe for re-use
or discharge into the environment.
Thanks to on-site water treatment, Alandale
Logistics has been able to halve the number of
sweeper vehicles it is using on the £3bn Chelsea
Barracks redevelopment, reduce the amount of waste
by more than 50% and at the same time, improve its
clean-up of the 12.8-acre site.
Alandale s was employing two sweeper vehicles to
keep the site and public roads clean. Due to London
traffic, each sweeper was spending up to three hours,
Thanks to on-site water treatment,
Alandale Logistics has been able
to halve the number of sweeper
vehicles it is using on the £3bn
Chelsea Barracks redevelopment