'); document.writeln('Elmhurst Sewage Treatment Works

HUMUS TANK MODIFICATIONS'); document.writeln('

'); document.writeln(''); document.writeln('BACKGROUND
Uneven brick outlet weir causing short-circuiting of flow through tank.(see image 2)Elmhurst STW is a small sewage treatment works, population 100, located just outside Lichfield in Staffordshire. It is of conventional design consisting of a single primary tank, pump station, biological filter and a small rectangular humus tank.(see image 1)There is no clean water or washwater available on this site; access is by a narrow farm lane only just wide enough for the 3,000 gallon tanker which is used to de-sludge the humus tank on a monthly basis.
Under normal flow conditions up to 1.13l/sec the works produces a good quality final effluent, however, during high flows encountered toward the end of monthly emptying schedule, solids could be washed out from the humus tank.
The final effluent sample point at Elmhurst STW is located on site a short distance from the humus tank, and consists of a shallow chamber under a lockable lightweight cover through which runs a horizontal half-round clay pipe of 100mm diameter.
Any solids washed from the humus tank could be deposited in and remain in the sampling chamber, putting at risk the compliance of any samples taken by the Environment Agency.

Works Consent: B.O.D.- 50mg/litre
SS: - 90mg/litre'); document.writeln('

'); document.writeln('ACTION:
Having identified the above problems on the site, the local Severn Trent management team asked Quatroserve to attend a site meeting to discuss solutions to the above problems, identify any further related problems and appropriate solutions for them.'); document.writeln('

'); document.writeln('RESULTS OF THE INVESTIGATION
PROBLEM 1
No scumboard fitted to outlet end of humus tank, so no retention of any floating solids.(see image 2)'); document.writeln('

'); document.writeln('SOLUTION 1
Fit HDPE (high density polyethylene) scumboard on tank outlet.(see image 3)'); document.writeln('

'); document.writeln('PROBLEM 2
Uneven brick outlet weir causing short-circuiting of flow through tank.(see image 2)'); document.writeln('

'); document.writeln('PROBLEM 3
Wide, flat, outlet channel which allows solids build up in low flow area’s.(see image 2)'); document.writeln('

'); document.writeln('PROBLEM 4
Effluent sample point poor, no freefall, solids likely to be scraped into sampling container during sampling.'); document.writeln('

'); document.writeln('SOLUTIONS 2/3/4
Fit "V" notch HDPE outlet weir plate located and sealed onto existing outlet wall.
"V" notches were arranged to be 70mm above the existing brick weir.This 70mm rise was made available because the tank inlet had a fall of some 100mm from the outlet at the base of the biological filter.( this ensures no backflooding of the filter drainage pipework, and associated restriction of air movement through the filter.

Fit a narrow HDPE channel just below new outlet weir "V"notches, with a gentle slope to the centre of the weir plate where a narrow "V" form spout collects both flows,
These flows are delivered to a central point in the tank outlet channel directly opposite the tank outlet pipe. (see image 4).'); document.writeln('

'); document.writeln('This results in a freefall for the effluent flow of approximately 125mm. A circular cut out in the new mesh flooring allows safe and efficient sampling of the final effluent.(see image 5).'); document.writeln('

'); document.writeln('PROBLEM 5
The existing mesh in the humus tank was of the diamond shaped "EXPAMET" type of approximately 2"x1" mesh size.This mesh was formally a support structure for the original "BANKS" clarifier pebble system.
Any rising solids rose straight through this mesh and onto the tank surface.(see image 2)'); document.writeln('

'); document.writeln('SOLUTION 5
To overcome the rising solids problem a double sided woven polyester mesh of 1200 micron size fitted to HDPE frames 0.75metre square and 80mm spacing was installed above the existing "EXPAMET" mesh.
The frames were constructed from 25mm HDPE and sealed to each other, and to the tank walls. (see image 6)'); document.writeln('

'); document.writeln('PROBLEM 6
Manual handling of heavy, solid mild steel chequer plate covers. These covers needed to be removed for access for the tanker suction pipe, and for the works operators to wash the tank walls and mesh.'); document.writeln('

'); document.writeln('SOLUTION 6
The heavy covers were replaced with medium duty galvanised steel mesh flooring panels, sized to enable smaller lighter sections to be positioned at each end of the tank.Three circular cut outs in the flooring allow access to:
A, effluent sample point,
B, 200mm galvanised pipe at inlet for tanker suction.
C, 100mm galvanised pipe at outlet for washwater suction.
(see image 5)above'); document.writeln('

'); document.writeln('PROBLEM 7
Existing steel access ladders could encourage unauthorised access to the humus tank.'); document.writeln('

'); document.writeln('SOLUTION 7
Ladders removed during installation of meshes.'); document.writeln('

'); document.writeln('PROBLEM 8
No washwater on site for tank cleaning, existing system relies on operators saving final effluent in buckets and swilling tank from above.'); document.writeln('

'); document.writeln('SOLUTION 8
1" Portable petrol pump and hoses supplied under contract. Method of use is for tanker to remove liquid from tank until level is just below mesh.
Operators then lower pump suction pipe from above into 100mm galvanised pipe under surface of remaining liquid.
Mesh and tank can then be hosed from above using portable pump until clean. Tanker then removes remaining liquid from humus tank.'); document.writeln('

'); document.writeln('FUTURE PROBLEMS
In order to prevent problems of algal growth on the mesh or surrounding tank structure, lightweight black HDPE covers were fitted to prevent sunlight entering the tank. These were secured using the existing locking bar system.(see image 7)'); document.writeln('

'); document.writeln('SUMMARY
There are numerous small rural sewage works of this type all across the UK , which may have some or all of the problems identified at Elmhurst STW.
Quatroserve has the ability to provide a cost effective package of solutions to overcome these and other problems associated with all types of sewage works.'); document.writeln('

'); document.writeln('THANKS
Our thanks to Severn Trent Water Ltd for permission to publish this feature.'); document.writeln('

'); document.writeln('ADDENDUM
The operation of the works has been improved by the installation of a small above ground holding tank, into which final effluent is pumped during routine operational visits. This volume of liquid is used to wash down mesh and tank floor during the humus tank cleaning sequence.'); document.writeln('