MBR AND RO TREATMENT OF DAIRY WASTEWATER: INDUSTRIAL WATER
RECYCLING OPPORTUNITIES
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2
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3
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B. Chapman , N. Goodman , T.H. Muster , S. Toze , L. Hodges , J. Sellahewa
1. ADI Systems Asia Pacific, Dunedin, New Zealand
2. CSIRO Land & Water Flagship, Clayton, VIC, Australia
3. CSIRO Land & Water Flagship, Dutton Park, QLD, Australia
ABSTRACT
Water is a valuable resource, and commitment to
environmental sustainability calls for innovative
water recycling opportunities. Anaerobic treatment
of food and beverage production wastewater results
in the production of biogas, a renewable source of
energy. However additional steps are required to
enable water recycling. A membrane bioreactor
(MBR) and reverse osmosis (RO) pilot trial, coupled
with ultraviolet (UV) and chlorine disinfection,
delivered a high quality potable water from
anaerobic effluent. The MBR achieved 97% COD
and 95% nitrogen removal, producing permeate
suitable for direct treatment in the RO. The RO with
UV and chlorine disinfection achieved potable water
criteria.
INTRODUCTION
Food production is the largest manufacturing sector
in Australia, and consumes large amounts of fresh
water. Even factories with a commitment to
environmentally sustainable practices through
implementing water, energy and waste assessment
programs and resource efficiency processes
produce wastewater as a by-product. In a dairy
factory, these water sources include condensate
from the evaporation of skim milk and other
products, and membrane filtration permeates
amongst other streams. The wastewater contains
not only organic pollutants (measured as COD), but
also nutrients such as nitrogen and phosphorus.
Various treatment options are available, However
the used of anaerobic technology results in the
production of biogas, a renewable source of energy
(Gant et al., 2002). It is important that the correct
type of anaerobic reactor is selected to match the
wastewater parameters, and in this case a full-scale
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low rate ADI-BVF reactor was installed and has
successfully treated the dairy effluent for many
years. The biogas generated is captured, and used
to operate a hot water boiler.
While the anaerobic reactor successfully reduces
the organic pollutants, the levels of nutrients such
as nitrogen and phosphorus do not change
significantly. Therefore while further treatment steps
offer the ability to generate a reusable water
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stream, they also offer a reduced cost of discharge
to sewer with a reduction in flow as well as nitrogen
and phosphorus load.
Biological nitrogen removal can be achieved in
aerobic treatment systems through the use of
alternating anoxic (low oxygen) and aerobic
sections. In the aerobic section the bacteria use the
available oxygen to convert ammonia (NH3) to
nitrate (NO3), which is known as nitrification. In the
anoxic stages a different group of bacteria convert
nitrate to nitrogen gas (N2), which is released into
the atmosphere, in a process called denitrification.
A readily available carbon source is required for
denitrification to occur.
Phosphorus can be removed biologically in aerobic
systems, through modifying the process to enable
bacteria to store phosphorus. Alternatively there are
various chemical precipitation methods which can
be employed, either within other treatment
processes or as a standalone step.
A membrane bioreactor (MBR) system is an
aerobic process, using modified activated sludge
technology to treat wastewater. A physical
membrane barrier is used to retain the aerobic
biomass within the treatment plant, rather than
gravity settling or other liquid/solids separation
techniques. The membrane effectively filters the
treated water, which results in a very high quality
final effluent with low suspended solids (often below
detection levels) and low organic content (<5 mg/L
BOD). The filtering reduces the need for external
disinfection operations because bacteria are
retained by the membrane, and also means that all
the biomass is retained in the reactor. This results
in the decoupling of the hydraulic and solids
residence times, as the solids are retained in the
system while water passes through. This means the
system can have a high hydraulic load rate, and
consequently a small reactor size/footprint, while
generating a treated effluent that can be directly
treated in a RO unit.
It is important to have low phosphorus in the MBR
system to manage the formation of calcium
phosphate precipitation and scaling which results in
inorganic fouling of the membranes, and also to
have low nitrogen levels for reverse osmosis.
While reverse osmosis removes almost all
pathogens, as well as salts and nutrients, final
disinfection of the treated water to control residual
microbial activity is essential. Potable water is
typically treated with UV disinfection and sodium
hypochlorite addition in a combined disinfection
process, which has been shown to be more
effective than either individually.
Here we report on a pilot trial demonstrating that
membrane bioreactor (MBR) and reverse osmosis
(RO) treatment of anaerobic effluent, coupled with
ultraviolet
(UV)
disinfection
and
sodium
hyphochlorite dosing, produce a high quality
potable water which is suitable for recycling within a
food manufacturing process.
EXPERIMENTAL CONDITIONS
The pilot system flow diagram is shown in Figure 1.
The MBR consisted of an 800 L pre-anoxic tank,
1,100 L aeration tank and 600 L post-anoxic tank
prior to the 340 L membrane tank. The two-stage
denitrification process promotes high levels of
nitrogen removal. Mixed liquor from the aeration
and membrane tanks was recycled to the preanoxic tank, recycling the biomass.
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The ADI-BVF anaerobic reactor effluent was used
as a feed for the MBR reactor. Chemical
precipitation was used to reduce the phosphorus
level prior to the MBR (Goodman et al., 2015), and
the average wastewater parameters following
phosphorus precipitation are shown in Table 1.
Untreated (raw) wastewater from the factory was
used to provide a carbon source for denitrification in
the anoxic zones of the MBR. The average
parameters of the raw wastewater are shown in
Table 1.
Table 1: MBR influent stream parameters
Parameter
COD (mg/L)
Total
Suspended
Solids (mg/L)
Volatile
Suspended
Solids (mg/L)
Phosphorus-P
(mg/L)
Total Nitrogen
(mg/L)
AmmoniacalN (mg/L)
160
Raw
Wastewater
4,800
230
1,015
80
940
45
60
175
155
135
55
MBR Feed
The membrane tank contained 10 full-size type 510
Kubota membrane cartridges. The design mixed
liquor concentration was 10,000 – 15,000 mg/L
suspended solids, with a maximum design flow rate
of 2,500 L/day.
The permeate generated by the MBR was treated
directly in the RO unit, which contained a single 4"
x 38" Koch 3838 HRX-VYV polyamide spiral wound
RO membrane (active area, 5.7 m²).
The RO pilot plant consisted of an inlet filter, feed
balance tank, high pressure pump with variable
speed drive and membrane housing, with a
backpressure control valve. The plant was run with
an operating pressure of 2000 kPa and a fixed flow
rate of 64 L/min.
A WaterTec UV2A disinfection unit, designed to
treat a maximum flow rate of 6 L/min was used for
UV disinfection followed by sodium hypochlorite
addition. Sodium hypochlorite dosing was achieved
using a metering pump and a 0.13 %w/w stock
solution of sodium hypochlorite to achieve a final
free chlorine residual between 0.5 and 2 mg/L.
RESULTS
The MBR demonstrated excellent biological
performance, achieving an average of 97% COD
removal, resulting in less than 40 mg/L COD in the
MBR permeate (Figure 2).
The plant also attained on average 95% nitrogen
removal, resulting in an average total nitrogen
concentration in the permeate of 21 mg/L. The
nitrogen removal efficiency showed continued
improvement through the trial, as the slow growing
nitrifying bacteria increased and reached a level of
maturity suitable for high levels of nitrogen removal.
As a result, a total nitrogen concentration of less
than 10 mg/L was achieved in the final two weeks.
In addition to the high level of COD and nitrogen
removal, the MBR permeate contained suspend
solids concentrations that were below detection
levels (data not shown).
The RO process received MBR permeate directly,
without any other treatment steps. The single pass
system produced re-usable quality RO permeate at
up to 50% recovery rate. Above 50% recovery, the
permeate quality was reduced due to elevated total
dissolved solids, making it less suitable as recycled
water (Figure 4).
Microbiological testing of the treated water following
UV and sodium hypochlorite disinfection detected
no presence of coliforms (E. Coli) or Enterococci,
indicating the water was potable quality. The water
quality met or exceeded the Australian Drinking
Water Guidelines on all parameters (Table 2).
Table 2: MBR effluent stream parameters
Treated
Water
Parameter
Suspended
solids (mg/L)
Total
dissolved
solids (mg/L)
Conductivity
(μS/cm)
Turbidity
(NTU)
pH
Total alkalinity
(mg/L)
True colour
(Pt/Co)
Sulphate as
SO4 (mg/L)
COD (mg/L)
Total Kjeldahl
nitrogen
(mg/L)
NO3-N (mg/L)
NO2-N (mg/L)
Total nitrogen
(mg/L)
Total
phosphorus
(mg/L)
Chloride
(mg/L)
Transmission
at 254 nm (%)
Calcium
(mg/L)
Magnesium
(mg/L)
Sodium
(mg/L)
Potassium
(mg/L)
Total
Hardness (mg
CaCO3/L)
Iron (mg/L)
Manganese
(mg/L)
Aluminium
(mg/L)
Required
Level
<0.1
35
500
58
0.2
5
6.5
6.5 – 8.5
9
<2
15
<2
8
<0.01
0.08
<0.001
50
3
0.08
0.05
9.7
250
100
1.7
0.03
8.2
1.3
4.5
<0.01
0.3
<0.01
<0.05
<0.01
<0.1
DISCUSSION
The goal of the MBR system was to remove the
majority of the COD, nitrogen and suspended
solids, generating a permeate suitable for treatment
by RO without additional pre-treatment steps. This
was achieved, as shown in Figures 2 – 4.
For this project chemical precipitation of
phosphorus prior to the MBR was selected. This
was due to biological phosphorus removal being
considered unsuitable, as the waste sludge would
be recycled to the existing anaerobic reactor. Under
anaerobic conditions, all biologically stored
phosphorus is rapidly released, resulting in an
increase of overall phosphorus levels in the
treatment plant. Biological nitrogen removal,
however, has no such issues and the two stage
denitrification process was shown to be highly
effective for nitrogen removal particularly once the
nitrifying population had matured.
The very high permeate quality, with low COD and
nitrogen levels, and non-detectable suspended
solids, was suitable for treatment by reverse
osmosis without any additional polishing steps. As
such, the MBR provides a single treatment step
contributing to a simpler overall process for
generating potable water.
MBR technology can also be used as a stand-alone
treatment for dairy wastewaters, however the
energy balance of combined anaerobic/aerobic
technologies is more favourable. The high quality of
the MBR permeate also allows water reuse in some
applications (non-potable), without the need for the
additional RO, UV and chlorination steps. Overall
treatment system capital and operating costs
should be evaluated on a case by case basis in
relation to the water reuse requirements for a
particular site.
The RO successfully generated a high quality
permeate suitable for re-use. The intended end use
of the permeate determines the degree of treatment
required. For potable water, the Australian Drinking
Water Guidelines ‘Total Dissolved Solids’ Fact
Sheet recommends 0-600 mg/L total dissolved
solids. This was achieved or exceeded, as long as
a recovery rate of 50% or less was applied in the
single pass RO membrane (Table 2). To produce
water with greater recovery, and retain potable
quality, a two-pass RO system should be
employed. The results indicate operating at 70%
recovery would not require antiscalants or
increased cleaning with this MBR permeate as a
feedstock.
The final disinfection steps of UV followed by
sodium hypochlorite dosing successfully removed
harmful microbiological contaminants, ensuring the
waste generated was of potable quality.
CONCLUSION
The biological performance of the MBR was
excellent, with high rates of both COD and nitrogen
removal demonstrated. The quality of the MBR
permeate was suitable for direct treatment in the
RO plant. The results indicate a full-scale single
pass RO plant could operate with 50% recovery, or
70% recovery with a two-pass RO system. The final
stage disinfection by UV and sodium hypochlorite
ensure the water quality meets the potable
standards, including microbiological safety.
ACKNOWLEDGMENTS
The authors acknowledge co-funding from the
Australian Water Recycling Centre of Excellence
through the Australian Government’s National
Urban Water and Desalination Plan, along with
CSIRO, Dairy Innovation Australia Ltd, and ADI
Systems Asia Pacific, as well as contribution of site
staff to the operation of the pilot plant.
REFERENCES
Australian Drinking Water Guidelines 6 (2011)
Version 2.0 – Total Dissolved Solids Fact Sheet.
Goodman, N., Muster, T.H., Chapman, B., and
Sellahewa, J. 2015. Phosphorus recovery from
dairy wastewater: A pilot trial. Proceedings of
Ozwater 2015, Australian Water Association.
Grant, S., Landine, R., Wilson, D., Molina, J.,
Norton, S., Qiu, Z., and Cocci, A. 2002. LowRate Anaerobic Treatment of Dairy Processing
Wastewaters. Proceedings of VII Latin American
Workshop and Symposium on Anaerobic
Digestion.
National Health and Medical Research Council.
Australian Drinking Water Guidelines, Version
2.0, Australian Government 2013.
MBR Feed
Preanoxic
Recycle
Postanoxic
Aeration
Recycle
Membrane
Reverse
Osmosis
UV/Cl2
Waste Sludge
Retentate
Potable Water
Raw Wastewater
Figure 1: Flow diagram of pilot plant system
Figure 2: Average COD concentration of the combined influent streams (raw dairy wastewater, and
phosphorus reduced anaerobic effluent) and MBR effluent COD concentration, showing removal
efficiency.
Figure 3: Total nitrogen level in the combined MBR influent streams, and the MBR permeate. The
nitrogen removal efficiency is also shown.
Figure 4: RO permeate conductivity in relation to system recovery.