Case Study SA-JW3
ANAEROBIC DIGESTER MIXING (VERTICAL MIXER)
Anaerobic digestion efficiency is directly related to the success of the mixing equipment that brings
microorganisms in contact with the substrate under optimised conditions. This case study explores
early results after switching from conventional mixing systems to vertical linear motion mixing.
Digester mixing optimisation for energy efficiency
Description of Process:
The digester mixing system consists of a flat horizontal plate fixed to the end of a metal shaft which
is attached to a cam, driven by an electrical motor. As the cam slowly rotates, the rod and horizontal
plate rise and fall, causing mixing of the digester content. The rod and plate are positioned inside the
digester while the cam and motor rest on the roof. The hydro-disk throws out an upward moving
liquid core on each downstroke, and a downwards moving core of liquid on each upstroke. These
high energy cores dissipate into the slurry and produce a steady bulk circulation in the reactor. The
disk is oscillating, but the majority of the liquid in the reactor does not oscillate but circulates in
response to the directional movement from the hydro-disk.
The advantage is that the energy consumption for a given mixing performance is a small fraction of
the consumption using conventional mixers, as result of the radial/axial circulation.
Potential Interventions

Fitting to existing digesters can be done by basic reverse engineering. New digesters can be
sized and fitted during design stage to adopt vertical mixers.

Optimise the digesters for maximum biogas output and methane yield by ensuring that good
process control practice is in place, which include monitoring the minimum mixing cycles
required to maintain
Range of potential Savings
Cost saving is estimated at R 830,000/year per digester, based on 350 kWh/day saving in energy.
Ultimately Johannesburg Water will be operating 30 sludge digesters on their various works.
Conservatively, it is estimated that a 50-60% power consumption reduction can be achieved using
the mixing with the radial/axial circulation, as opposed to rotary fluid motion mixing.
The following table summarises the performance targets set of VLM vs. industry standards:
Type of mixing device
Unit power rating
3
Vertical linear motion mixing
Impellor draft tubes
Mechanical mixing
Gas draft tubes
kWh/m
1.316
12.112
5.226 – 2,330
5.266 – 7.899
Case Study SA-JW3
Hp/1000cL
0.05
0.46
0.2 – 1.0
0.2 – 0.3
ANAEROBIC DIGESTER MIXING (VERTICAL MIXER)
Ref
Case Study SA-JW3
Location:
1
Sector:
2
Response information, description and remarks
Johannesburg Water is situated in Central and Southern Gauteng
Province of South Africa. The areas are densely populated. Two of
the works are situated in the northern catchment and three in the
southern catchment.
Sewage sludge
Works Owner or Operator:
The plants are owned by the City of Johannesburg (Water Services
Authority) and operated and maintained by Johannesburg Water
(Water Services Provider).
Size:
Some 3.5 million people are served in the northern catchment and
2.5 million people in the southern catchment.
Energy Provider:
Power is in form of electricity, provider by the national
electricity agency, ESKOM. Plant requires 8.0 MW energy, with
potential to generate 4.4 MW electrical energy and 110.8
MWh/d
heat.
Operation
consists of a plate fixed to a metal rod that protrudes
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Process:
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through the digester roof. The metal rod is connected to a cam
which is motor driven.
Component:
All waste activated and fermented raw sludge is digested. This case
study refers to the anaerobic digestion section of the plant only.
Motivation for the case study:
Most sludge treatment and disposal options available require larger
amounts of electrical energy to function. This process requires only
a small electrical motor to drive the cam.
Process/Plant changes:
The opening in the roof of an existing digester needs to be widened
to receive the mixing plate. The cam and motor are fitted to the
roof above the mixing system.
Civil/Physical Changes:
Widening of the opening in the digester roof.
Operational Changes:
Reduced maintenance requirements and no changes in skills level
are required.
Risks and Dependencies:
The operation has been tested in the USA over a period of about 8
years and has been in operation in Johannesburg for 12 months.
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Implementation:
The project was procured via a design/build process with a
respectable contractor. Acceptance testing, followed by an O&M
period was included as part of the contract.
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Energy Efficiency gains:
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Cost/Benefit analysis:
No cost benefit analysis has been done.Cost saving of R 83,000 per
annum per digester is estimated@ R 0.65/kWh.
Project review:
There appears to be no reason to consider any improvement to the
system at the present time.
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It is expected that there will ultimately be a reduction in electrical
energy use of about 10 500 kWh per day in Johannesburg for the
total sludge digestion operation (350 kWh per day per digester)
Confidence grade:
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Low – work in progress and performance need to be monitored
over a 2 year project cycle. The main concern that Johannesburg
has at present is failure of the plate fixture to the metal rod. This
will need to be inspected when the digester is emptied for cleaning
out of any settled material.
Johannesburg Water is in pursuit of full compliance to legislation which directs quality of sludge
disposed to the environment for availed for down end users. The quest for optimised and more
efficient digestion commenced with the refurbishment of the existing digesters at the wastewater
treatment works, which involved the replacement of the digester heating and mixing systems.
Sludge to water heat exchangers were the preferred heating system however, the WEFTEC
Conference in Chicago, USA (2010), promoted innovative energy saving digester mixing system as
tested at a wastewater treatment plant in Arizona. At the time, it was claimed that 50% of the
electrical energy could be saved in digester mixing by this system.Johannesburg was due to build
new digesters at two of their wastewater treatment works viaDriefontein and Bushkoppie. The
tender for the digester heating equipment for Driefontein Works allowed for alternative offers and
the missing systems were subsequently procured.
Description of Process
In the past, Johannesburg had installed both gas and jet mixing systems in digesters and while jet
mixing was the preferred option, the system required high electrical energy use. Due to the present
rapid increase in electrical energy cost, it was decided to install vertical linear motion mixers.
The digester mixing system consists of a flat
horizontal plate fixed to the end of a metal shaft
which is attached to a cam, driven by an electrical
motor. As the cam slowly rotates, the rod and
horizontal plate rise and fall, causing mixing of the digester content. The rod and plate are
positioned inside the digester while the cam and motor rest on the roof. A gas seal is fitted to the
opening in the digester roof through which the rod passes and prevents biogas from leaking out into
the atmosphere. The plate only moves 400 mm in the vertical plane, which is sufficient for the
digesting sludge to mix efficiently. The Driefontein mixing system has been in operation for 12
months without any major operational problems. The volume of each of the two digesters is 2500
m3 and requires a 7.5 kW motor to drive the cam. The recently renovated digesters at Northern
Works are each 2000 m3 and require 22 kW motors to pass recycled sludge through the jet mixers.
Due to the success of the Driefontein installation, it was decided (2011) to install the Enersave
mixing system in the new digesters being constructed at the Bushkoppie Works and in any future
digesters that require refurbishment.
Potential Interventions

Periodic cleaning of the digesters becomes easier since the digester is clear of any pipework or
draught tubes except for the plate which is some 2/3 off the floor.
The maintenance requirements are reduced and only minor attention need to be given to the
cam and motor.
The Enersave system requires less than 50% of the electrical energy requirements of other
mixing systems.


Cost Saving
Ultimately Johannesburg Water will be operating 30 sludge digesters on their various works.
Conservatively, it is estimated that a 50-60% power consumption reduction can be achieved using
the mixing with the radial/axial circulation, as opposed to rotary fluid motion mixing. The following
table summarises the performance targets set of VLM vs. industry standards:
Type of mixing device
Unit power rating
3
Vertical linear motion mixing
Impellor draft tubes
Mechanical mixing
Gas draft tubes
kWh/m
1.316
12.112
5.226 – 2,330
5.266 – 7.899
Hp/1000cL
0.05
0.46
0.2 – 1.0
0.2 – 0.3
The energy saving from a single application at the Driefontein and Bushkoppies plants are:


350 kWh per day per digester
Cost saving of R 83,000 per annum per digester (@ R 0.65/kWh).
The collective improvement of all 30 digesters operating with vertical mixers will be (work in
progress):

reduction in electrical energy use of about 10 500 kWh per day in Johannesburg for the total
sludge digestion operation.
References:

Shaun Deacon, Wastewater Engineer, Johannesburg Water.