Applications30

WORLD PUMPS October 2013

www.worldpumps.com

Innovation in mixing

maximises AD process

Water & wastewater

Helping the municipal waste and AD

industry by providing eff ective solutions

to the problems of maintaining and

maximising the performance of digesters has

been a key objective for System Mix Ltd. Since

2003 Cambridgeshire-based System Mix, in

conjunction with P&M Pumps, has developed

the concept of pumped mixing systems.

A fundamental part of this success is due to

its ability to design and supply complete

bespoke mixing systems. Supplying not just

the hardware, but to design, innovate and

provide assurances based on experience

gained in the field, gives prospective

customers the confi dence to proceed with

projects without fear of losing performance

and capacity, which is so often seen following

the initial phase of a process plant’s life cycle.

The types of feedstock available to non-munic-

ipal AD operators are quite diverse and this calls

for a fl exible and open approach to equipment

selection. The company provides a case-by-

case analysis of their customer’s requirements

based on specifi c feedstock characteristics such

as solids content, (biological and non-biolog-

ical), chemical imbalances likely to cause

foaming and the ability to handle both deposi-

tion and fl oating scum simultaneously.

The combination of standard and bespoke

pump and nozzle combinations in diff erent

materials and confi gurations provided by

System Mix are designed to accommodate

existing and new build structures of varying

volume and aspect ratios. This provides AD

plant designers solutions to problems associ-

ated with other mixing technologies. This

Anaerobic Digestion (AD) operators inevitably want to achieve higher gas yields, reduce foaming

and also eliminate the potential for maintenance problems. Furthermore, in today’s economic

climate, it’s important for operators to fully maximise the sustainability benefi ts of their process

plant which means each part of the AD process is under even closer scrutiny. System Mix can help

with all of these objectives.

The Vaughan Chopper pump on the exterior of the tank with the dual zone mixing pattern indicated.

Applications 31

WORLD PUMPS October 2013

www.worldpumps.com

which act as scum layer breakers and non-

chemical foam suppression -- Scumbuster

and Foambuster. Foambuster uses the

digester to spray coarse heavy droplets onto

the foam, either continuously, or as required,

using either a dedicated chopper pump or

nozzle. This can also be achieved by

designing the system to accommodate the

additional fl ows and pressures required using

the existing Rotamix mixing system with the

pump operating with a variable speed drive

to maintain best effi ciency. This has given AD

operators the benefi t of being able to apply

a virtually universal solution to the problem

of how to eff ectively suppress foam.

One such pilot scheme required System Mix

to provide a trial system for evaluation on

one of fi ve digesters at Anglian’s Water,

Great Billing sewage treatment works. As

this particular digester incorporated three

roof-mounted nozzles it was decided to

modify one of these by reducing its length

and adding a plume distributor plate,

approach usually incorporates the basic

Rotamix concept of dual zone mixing.

The ability to maintain a healthy, high yield

digester can be aff ected by unstable biolog-

ical issues that are often complicated by

resulting issues, such as the formation of

foam. It is likely that excess, valuable gas

production has to be curtailed when gas

creates a foaming situation and this is often

attributed to over mixing and \ the only way

to prevent this is to de-tune the digester.

In digesters where the feed material is quite

benign, foaming is likely to be more easily

controlled. Similarly, where the feed material

varies in its volatility, steps need to be taken to

quickly control the damage foam can cause

both to process and in some cases, the

equipment.

In addition to the Rotamix fl oor-mounted

nozzle systems, System Mix now also incor-

porate (optional) additional high level nozzles

allowing around just over 10% of the recir-

culation fl ow to act as the foam buster. It

was important for this work to be carried

out without the need to empty the digester

and with minimum impact on Great Billing’s

overall sludge handling capacity. In extreme

circumstances during 2012 this digester

consumed a one cubic metre capacity IBC

of de-foamer every two or three days.

Following the installation of the trial system

Anglian Water has calculated that this new

arrangement has resulted in savings of

approximately £20,000. This equates to a

payback of less than two months of their

investment. With plans to install similar foam

suppression nozzles on the other four

digesters at Great Billing, it is estimated that

under normal operating conditions, the

return on investment will be just six or seven

months.

The continuous development and under-

standing of the digestion process and with

sludge rheology, plus a wealth of under-

standing of solids pumping and mixing,

provides AD system operators with the confi -

dence that they can install the Rotamix System,

without fear of expensive downtime and poor

performance. Furthermore, by providing addi-

tional process performance guarantees,

backed by CFD analysis (computerised, fl uid

dynamics) it is possible to show that more than

95% of available working volume can be

utilised ensuring that high volatile solids

destruction and therefore gas production, is

maintained through the life of the digester. It

demonstrates that when a comparison is made

between a real time tracer test and a simula-

tion on a working digester there are close simi-

larities. This offers further savings and

assurances that the requirement for expensive

and sometimes inconclusive testing may not

always be required.

www.pandmpumps.co.uk

The pipe work and nozzle confi guration on the base of the tank which produces the dual zone rotation technique.