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How to boost the biology in a municipal waste water treatment plant, a case study
1
How to boost the biology in a municipal waste water treatment
plant, a case study
Hans Wouters1, Farah Al-Massri2, Hans Mollen3, Jessica Gnako4, Malcolm E. Fabiyi4, Melanie
Nuchelmans3
1 Brightwork BV, the Netherlands, corresponding author: h.wouters@brightwork.nl
2 BW Products BV, the Netherlands
3 Waterschap Brabantse Delta, the Netherlands
4 Drylet LLC, United States
Key words: Sludge production, biological treatment, WAS reduction
ABSTRACT
A bioremediation pilot program was successfully executed at WwTW Halsteren, the Netherlands,
operated by the Dutch water authority Water board Brabantse Delta. The program was based upon
dosing proprietary Aqua Assist biocatalysts into the activated sludge reactor to boost the biology
with the primary aim to reduce the production of waste activated sludge.
The Aqua Assist solution relies upon the regular dosing of an engineered microporous silica oxide-
based dry-to-the-touch granular matrix with particle sizes in the range of 200 – 600 microns,
inoculated with a specific proprietary microbial population, directly into the activated sludge reactor.
Within the reactor the media will typically have a retention time equal to the sludge retention time.
The particles are protecting the microbes from predators, bacteriophages and/or toxins and act as a
nursery for the biomass to grow, with doubling times of 2 – 3 hours under more or less optimal
circumstances, once the material is dosed into the activated sludge basin. With a sludge retention
time of 19 days in the Halsteren activated sludge basin, the introduced Aqua Assist biomass roughly
contributes to at least 10% of the total biomass present. Studies have revealed that the Aqua Assist
dosing initiates a shift in bacteria species which favour the process conditions for wastewater
treatment.
The trial was performed between June and November 2019, dosing a small amount of Aqua Assist
twice a week into the activated sludge tank. It resulted in the increase of the MLSS concentration
from 2.7 g/l up to 3.7 g/l without any impact on SVI, effluent quality and plant operation stability.
The objective to reduce WAS production was achieved with a dry solids reduction of around 25%. As
a result, a positive business case was demonstrated and a follow-up of the project is initiated.
How to boost the biology in a municipal waste water treatment plant, a case study
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INTRODUCTION
Maintaining the optimal MLSS in a municipal wastewater treatment plant is important for both
treatment performance and energy efficiency. The more organic matter is consumed by the biomass,
the lower the biochemical oxygen demand (BOD) will be in the discharge. The goal of this Aqua Assist
pilot program was to minimize waste activated sludge (WAS) production by raising the MLSS to a
higher level without this interfering with the process of purification. At higher MLSS values, solids
retention times (SRT) are longer, enhancing the tendency for endogenous respiration of the
microbes. Endogenous respiration involves the consumption by microbes of internally stored carbon,
leading to lower sludge yields (Loosdrecht & Henze, 1999). A full scale trial was executed in the
Netherlands to verify the impact of dosing Aqua Assist. Tests were performed at WwTW Halsteren,
operated by the Dutch water authority Brabantse Delta, during 2019.
HOW DOES IT WORK?
For microbes to survive in a competitive environment within an activated sludge plant for
wastewater treatment and to contribute to the effective treatment, three main conditions must be
created. First, access to food needs to be facilitated – microbes obtain food from organic matter by
first adsorbing the COD molecules onto the outer surface of the cell, prior to the diffusion of the
molecules into the cell; a high surface area for adsorption promotes access to food. Secondly,
microbes must be protected from predation by other organisms and/or toxins. Finally, the
environment needs to enable microbes to be retained for extended periods of time in the treatment
system (refer to figure 2).
By dosing Aqua Assist particles, all three criteria are met by creating a favorable environment of
carefully selected bacteria. The solution relies upon the regular dosing of an engineered microporous
silica oxide based dry-to-the-touch granular matrix with particle sizes in the range of 200 – 600
micron, specific density of 590 kg/m3, inoculated with a specific proprietary microbial population, as
indicated in figure 1.
Figure 1 – Production process Aqua Assist
The biocatalysts are dosed directly into the activated sludge tank, with a typical retention time equal
to the sludge retention time. The particles protect the microbes from predators and/or toxins and act
as a nursery for the biomass to double every 2 – 3 hours under more or less optimal circumstances.
As the microbes proliferate within the pores of the particles, they are exuded into the bulk liquor.
How to boost the biology in a municipal waste water treatment plant, a case study
3
Figure 2 – Main drivers for Aqua Assist impact
The microbes which are loaded onto the inorganic silica microporous medium are characterized by
their high capability for efficient metabolizing, not being filamentous in nature but floc forming, and
enhancing hydrolysis with both lipophilic and halophilic functionality. With regards to their oxic
preferences, the microbes are facultative and aerobic.
The ideal growth rate of the microbes in the granular environment will have a relevant impact on the
total biomass present in the activated sludge basin. This impact has been calculated for the
Halsteren plant with a total activated sludge volume of 2,580 m3, an average MLSS content of 9,000
kg and a SRT of 19 days. 1 kg of product contains 50 g of microbes (5% by mass), with a maximal
growth potential in the secured environment. During the test period with Aqua Assist dosing (April –
November 2019) a daily dosage of 600 g of product was applied, equalizing the addition of 30 g of
microbes per day. Although the dosage is low, the high sorption capacity of the porous media and
the protective properties that it provides enable the microbes to enter rapidly into an exponential
growth phase. With typical doubling times of about 2-3 hours, a yield is established of 50-100 kg of
biomass within a 24-hour timeframe.
The applied technology should not be confused with biomass growth on top of carriers in Moving
Bed Biological Reactor (MBBR) systems, as the mechanism for growth is different. In an MBBR
system, biofilm growth starts from outside in. The microbes are not carefully selected but are already
present in the system and are only using the carrier to grow and retain inside the tank. In contrast,
the microporous Aqua Assist media carries carefully pre-selected, embedded microbes.
Changes in microbial population due to Aqua Assist dosing
The microbial culture was studied in some activated sludge plants located in Massachusetts, USA
before and after dosing Aqua Assist to find out whether and, if so, how the shift of microbial
population occurs. In the table below, the most relevant microbial species found in the activated
sludge tank are tracked before and after regular dosing over the course of a year.
The results indicate that a clear shift is taking place, with particular species growing strongly while
other species tend to decrease after dosing. This shift contributes to growing a population of
microbes which are favourable for the biological processes in the wastewater treatment plant.
How to boost the biology in a municipal waste water treatment plant, a case study
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For both aeration tanks AT1 and AT3, table 1 lists the microbes with a relative abundance greater
than or equal to 2% in either the initial or final sample and have decreased or increased by 15% or
more after the use of Aqua Assist. As more factors are impacting microbial shifts in activated sludge
processes, these findings should be verified carefully.
Table 1 Predominant changes in microbial population before and after Aqua Assist dosing in a serial activated sludge scheme
at activated sludge plant located in Massachusetts, USA
Aeration tank AT1
Aeration tank AT3
% Abundance
% Abundance
Initial
Final
% change
Initial
Final
% change
Arcobacter sp
9.3
7.9
-15%
6.1
28.9
+370%
Comamonas sp
2.5
0.0
-100%
2.3
0.0
-100%
Curvibacter sp
2.2
3.6
+67%
3.2
2.2
-30%
Denitromonas sp
2.9
0.0
-100%
4.2
0.0
-100%
Flavobacterium sp
2.1
6.8
+221%
2.3
4.5
+98%
Haliea sp
6.3
0.0
-100%
5.6
0.0
-100%
Hoeflea sp
0.3
10.4
+3694%
0.2
10.7
+6682%
Hydrogenophaga taeniospiralis
3.4
0.9
-74%
3.3
0.5
-84%
Hyphomonas jannaschiana
3.9
0.2
-94%
4.1
0.1
-97%
Marimicrobium arenosum
3.8
0.0
-100%
3.9
0.0
-100%
Psychroserpens mesophilus
13.1
0.0
-100%
14.8
0.0
-100%
Rhodobacter sp
3.0
11.0
+271%
1.6
8.9
+471%
Vitellibacter nionensis
7.9
0.1
-99%
9.1
0.1
-99%
Sulfurimonas sp
_
_
_
0.4
2.4
+431%
Lewinella nigricans
0.7
8.6
+1086%
_
_
_
Thauera phenylacetica
2.1
0.0
-100%
_
_
_
PROOF OF PRINCIPLE AT WWTW HALSTEREN
WwTW Halsteren (figure 3), the Netherlands
is operated by the water authority Brabantse
Delta and is characterized as a low-load
biological treatment plant, with a capacity of
12,700 pe, a maximum hydraulic capacity of
525 m3/h. Phosphorus is chemically removed
by dosing aluminum into the aeration tank
(AT). WAS is thickened and stored prior to
transport to a central sludge digesting
facility.
Figure 3 – WwTW Halsteren
How to boost the biology in a municipal waste water treatment plant, a case study
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To analyze the effectiveness of dosing Aqua Assist, an initial comparison was made with the process
data from 2017, a year with more or less similar process operations.
The dosing started early June 2019 and was continued until the end of November 2019. Twice a week
(typically Monday and Thursday) during this period, a fixed amount of 2.1 kg of Aqua Assist was
manually dosed into the return sludge flow mixing with the feed water. This caused an ideal mixing
of the material.
Initially the MLSS was not changed and remained at 2.7 g/l. From the end of July onwards the MLSS
was increased over 120 days to a concentration of 3.7 g/l. A summary of the results is available in
Table 1.
Table 1 – Summary of pilot Aqua Assist dosing at WwTW Halsteren
Year
No.
of
days
Average
SVI
(ml/g)
Average
MLSS
(mg/L)
Total
sludge
discharge
(m3)
Total
WAS
flow
(m3)
Average
sludge
discharge
TSS
(kg/d)
Average
sludge
discharge
DS
(g/L)
Total
sludge
discharge
TSS
(kg)
Yield
based
on
BODfeed
(kg/kg)
Yield
based
on
CODfeed
(kg/kg)
2017
120
87
2,673
2.997
19,688
540
21.8
64,795
0.76
0.27
2019
120
91
3,650
2.676
12,492
426
19.1
51,101
0.55
0.21
In Figure 4, both the 2017 and 2019 data were plotted for MLSS and SVI, indicating no impact on SVI
after increasing MLSS concentrations. The combination of Aqua Assist dosing and the possibility to
run the plant at an increased MLSS concentration resulted in a decrease in sludge production of 25%.
This equals a decrease in the specific WAS production from 15.5 kg solids per pe per annum to 12.2
kg solids per pe per annum.
Furthermore, the effluent quality remained stable and good. No impact of rainy weather flow
conditions causing higher solids concentrations in the final effluent, which were recorded in the
reference year 2017, was detected in 2019.
How to boost the biology in a municipal waste water treatment plant, a case study
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Figure 4 - SVI and MLSS plotted in time
CONCLUSIONS
The pilot program at WwTW Halsteren resulted in a 25% reduction in WAS production, from 200 ton
dry solids per annum to 155 ton dry solids per annum. At the same time the operating conditions
remain stable and final effluent quality consistently meets the criteria. MLSS could be increased from
2.7 g/l up to 3.7 g/l without any process consequences. The effluent quality remained stable and
good throughout the piloting period.
An economically positive business case for deploying Aqua Assist has been demonstrated in this case
study. The cost for dosing the material is significantly lower than the annual savings on sludge
handling. By reducing the WAS production, the water authority is contributing to its objectives of
reducing waste production and achieving operational savings.
Ongoing research is scheduled, in which the mass balances will be considered in more detail and the
microbial analysis of the prevailing species will be taken into consideration.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
18-8-2016 6-3-2017 22-9-2017 10-4-2018 27-10-2018 15-5-2019 1-12-2019
MLSS MG/L
SVI(ML/G)
DATE(DAYS)
SVI 2017 SVI Pre-Drylet (2019) SVI Drylet 2019 MLSS Drylet 2019 MLSS2017
How to boost the biology in a municipal waste water treatment plant, a case study
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