THE USE OF ALGAL BIOFILMS TO ENHANCE DECENTRALIZED COMMUNITY
AND COMMERCIAL ON-SITE WASTEWATER TREATMENT
Daniel Johnsona and Louis Lefebvrea
Abstract
Algal biofilms leverage the symbiotic relationship between algae and bacteria to create a diverse
treatment ecology that is controlled, stable and delivers many treatment benefits. Bacterial
systems are dependent upon high levels of oxygen taken from the air for the oxidation of organic
matter. Since the by-product of photosynthesizing algal biofilms is oxygen, the costly demand
for external aeration is dramatically reduced.
Through co-evolution, algae (photosynthetic microbes) and bacteria have developed an intricate
relationship wherein the wastes of one group are the inputs for the other. Algae produce oxygen,
consume carbon dioxide, and exude polysaccharides. Both heterotrophic and autotrophic bacteria
consume oxygen, while heterotrophic microbes require organic carbon and produce carbon
dioxide as an input for the photosynthesis. Oxygen produced by the algae greatly facilitates
nitrification, as the algal biofilms are 100% saturated with oxygen during daylight hours 1.
This mutually symbiotic relationship creates a highly stable and sustainable environment for both
algae and bacteria to thrive. Conventional systems such as activated sludge require a population
of bacteria large enough to provide consistent treatment. This bacterial population is highly
dependent upon a consistent flow of carbonaceous biochemical oxygen demand (CBOD) and
thus the proper management of recycled activated sludge (RAS) to maintain the system. In
contrast, the algal biofilm requires no sludge recycle and produces polysaccharides that can be
used by the bacteria 2, delivering a buffer against both shock loading and diluted flow.
Algal attached growth technologies are particularly applicable to small community and
commercial onsite treatment applications for direct discharge or water reuse. Many of these
facilities are seasonal and receive highly variable flows and loadings, which make it very
difficult to consistently meet their permitted effluent limits.
Introduction
Algaewheel® technology using fixed film algae and bacteria offers many treatment benefits to
decentralized wastewater facilities. Decentralized treatment is a challenging endeavor because
of many factors; seasonality and variable flows, slugs of concentrated waste or diluted waste,
operational and maintenance commitments, and power consumption. These challenges can be
reduced and in many cases eliminated by incorporating algal biofilm technology.
Algal-bacterial fixed film treatment systems offer the operational benefits of long sludge
retention times and lower operational attention since there is no recycling of activated sludge to
maintain the appropriate mixed liquor volatile solids. Moreover the incorporation of the algae
into the biofilms provides many treatment benefits, which increase system stability. Traditional
fixed film systems are reliant on the diffusion of oxygen from the bulk water or from the
atmosphere though the boundary layer of the biofilm and through several layers of biofilm. This
limitation can result in anoxic or anaerobic biofilm conditions reducing nitrification rates and
1 CBOD removal. To counter this oxygen deficit, additional aeration or increased revolutions per
minute (rpm) of rotating biological contactors are used to increase oxygen transfer. This
increases both surface turbulence and brings biofilms in contact with the air more frequently
increasing gas transfer from the atmosphere.
In contrast algal biofilms produce oxygen within the biofilm, eliminating the oxygen deficit and
in fact providing surplus oxygen to the bulk water. Kuenen et al determined that oxygen levels
in photosynthetic biofilms on trickling filters reach 500% of saturation oxygen 1 Epping 3
measured net oxygen production of photosynthetic biofilms to be 40 nmols cm-1 min-1 and
Bernstein 4 measured net oxygen production at 18 nmols cm-1 min-1 in biofilms grown in
municipal wastewater. These oxygen production rates are equal to or greater than the typical
diffusion rate of oxygen through biofilms of 20 nmols cm-1 min-1. 3 Because of the high oxygen
output of the algae, additional aeration for biological wastewater treatment for both heterotrophic
and nitrifying bacteria is minimal.
Nitrification also decreases pH in the treatment system as H+ are liberated from ammonia during
the production of nitrate. Algal biofilms have the potential to reduce the acidification of the
biofilms and water. The photosynthetic reaction consumes CO2, which increases the pH. The
corresponding increase of both dissolved oxygen and pH are commonly observed in the system
showing that primary inhibitory factors to nitrification are easily remedied by coupling algal
photosynthesis with nitrifying bacteria.
Algal-bacterial interaction throughout the biofilm acts to increase diversity and generate a stable
community with consistent treatment in the same way biodiversity improves natural water
quality. 5 Studies have shown that there are many interactions between algae and bacteria that
extend beyond photosynthesis. These reactions include but are not limited to bacterial
consumption of algae, algal exudates and increased dissolved organic carbon (DOC) removal by
bacteria in the presence of algae. 6-9
Algaewheel® process:
The fixed film treatment system centers on the use of an air driven algal contactor, which is filled
with biofilm carriers. Both the external surface of the contactor and the carriers are substrates
for biofilm growth. The exterior of the contactor is exposed to ambient or artificial light,
encouraging the growth of phototrophic organisms (algae). This exterior biofilm is the source of
the algal derived oxygen and also the location of many of the autotrophic and heterotrophic
bacteria in the treatment system. The internal carriers primarily are covered in nonphotosynthetic biofilms, containing bacteria.
Each contactor rotates independently around a guide shaft as the aeration provides lift and
rotation. It is common for the contactors to rotate at varying speeds based on biofilm growth.
The contactors are sparged periodically in order to aid in the sloughing of “old” biofilms.
Bacterial and algal biosolids move through the treatment tank and are then collected in a
secondary clarifier. Biosolids production in the system is similar to that of typical aerobic
systems with the addition of the algal fraction produced by only the photosynthetic biofilms,
accounting for 5 g m-2 day-1 or a 2% increase.
2 Primary Clari-ier Algaewheel Process Secondary Clari-ier Disinfection Figure 1: Basic process flow diagram of an Algaewheel wastewater treatment system.
Materials and Methods
Water quality data in this case study were collected between November, 2011 and May 2014.
Over the period 92 Samples were collected, at a minimum, on the monthly basis but were
collected weekly for much of the sampling effort. Water samples were collected as 1 liter grab
samples and analyzed within 24 hours by Cardinal Labs (104 North St, Wilder, KY North St,
Wilder, KY 41071) according to Standard Methods for the Examination of Water and
Wastewater 10. Samples were analyzed for ammonia (NH3), carbonaceous biological oxygen
demand (CBOD), combined nitrate and nitrite (NOx), total suspended solids (TSS), and reactive
phosphorus (P).
Diversity estimates were determined by analysis of the 16s r RNA subunit. Biomass samples
were collected in January 2015 by scraping the surface of the 24 contactors in the primary
treatment tank. DNA was extracted from the samples using a FastDNA® SPIN Kit for Soil (MP
Biomedicals) and then purified using a GENECLEAN® Kit (MP Biomedical).
The V4 region of the 16S gene was amplified using V4 515F and V4 806R primers according to
a previously established protocol 11. The sequences were then analyzed using MOTHUR 12and
assigned taxonomic classifications using the Silva ribosomal database. 13
Cincinnati Nature Center Example Case
The Cincinnati Nature Center (CNC) installed an algal treatment system in 2011 to treat a daily
average flow of 1500 gallons and 3000 gallons peak daily flow. The system effluent flows into
an ephemeral receiving stream that passes through the park and meets surface discharge permit
limits of CBOD-10, TSS-12, and NH3-1. The CNC facility is operated and maintained by a
single employee, dedicating approximately 4 hours/week to operation. The operation includes
checking sludge levels in clarifiers, moving sludge to the sludge holding tank, preparing and
dosing sodium bicarbonate to increase alkalinity, and general cleaning and maintenance. The
CNC system is stressed by several common factors that complicate decentralized and onsite
treatment. Hydraulic loading to the facility varies seasonally and throughout the week based on
special events, school tours, and weather conditions. The waste stream at CNC is primarily toilet
and sink waste associated with the visitor center and park restrooms. There is a single residence
on the premises that does produce some kitchen and shower waste when interns are occupying it.
Additionally, mass loading and influent concentrations of ammonia and CBOD fluctuate
dramatically. These fluctuations result from collection system infiltration, special events, and
the periodic use of a residence onsite used for visiting interns. The use of the residence results in
decreased ammonia and CBOD concentration since the waste stream is diluted by water from
showers and sinks which has lower concentrations of NH3 and CBOD. This increased water
usage does increase hydraulic loading.
3 Figure 2: View of the system showing the algal biofilm.
The CNC system has consistently met the ammonia discharge limits with only three exceptions
from November of 2011-May 2014. All of these exceptions were a result of operational errors
related to the decanting of to much digester supernatant to the head of the system. The system
receives a wide range of NH3 loading ranging from 10 mg/l to 120mg/l.
Results
Over the course of the study the three parameters of concern were those subject to the national
pollution discharge elimination system (NPDES) permit. The average influent and effluent
parameters are reflected in Table 1.
Table 1: The average, minimum, and maximum chemical and physical permit parameters for CNC; CBOD, NH3, and
TSS.
Average
Std dev.
High
Low
CBOD5
(mg/L)
influent
CBOD5
(mg/L)
effluent
TSS (mg/L)
influent
TSS (mg/L)
effluent
NH3 (mg/L)
influent
NH3 (mg/L)
Effluent
123
2.3
229
4.2
52
0.16
58
1.1
113
2.2
18.1
0.3
268
3.6
480
11.6
120
2.34
37
0.2
76
1.8
19.5
0.03
Figure 3 below, demonstrates the system’s ability to perform despite greatly variable influent
ammonia concentrations. Ammonia limits were not met on three days over the three year study.
4 Figure 3: Ammonia influent and effluent concentrations at CNC from 2011-2014
Influent ammonia concentrations fluctuated between less than 19.5 mg/l and 120 mg/l during the
three year study period. Consistent effluent ammonia concentrations show the system’s ability to
process daily and seasonally fluctuating waste characteristics.
Similarly, Figure 4 demonstrates the system’s ability process widely varying influent CBOD
concentrations. There were no days the system did not meet the effluent CBOD limit over the
three year study period.
5 Figure 4: CBOD influent and effluent concentrations at CNC from 2011-2014 reflect wide variability of influent
concentration while maintaining consistent effluent quality.
During the study period influent CBOD concentrations ranged from below 10 mg/l to over 350
mg/l. Consistent effluent CBOD concentrations, below effluent limits, despite influent CBOD
concentration variation.
Total suspended solids removal was achieved in the system through the use of a primary and
secondary clarifier. The clarifiers accumulate solids from the raw waste stream and biosolids
form the algal system. The biosolids from the system are settled in the same manner as
traditional systems resulting in TSS levels below the NPDES permit level.
6 Figure 5: Influent and effluent TSS concentrations at CNC during the testing period.
7 The rotating contactors at CNC were examined microscopically and were predominantly covered
in the filamentous green alga Cladophora and to a lesser extent diatoms such as Gomphonena
and Nitzchia. The bacterial assemblage consisted of over 500 operational taxonomic units
(OTU) of bacteria and were detected by DNA sequencing of the 16s rRNA subunit. Figure 6
below, demonstrates the ten most abundant bacterial OTUs to the family level. The pie chart
provides a snapshot into microbial biodiversity that populates the system at CNC. In the CNC
study 10 OTUs (operational taxonomic units) accounted for 38% of the individuals sequenced.
The remaining 62% included over 500 additional OTUs.
Carnobacteriaceae 7% Xanthomonadaceae 7% Comamonadaceae 4% Unclass Bacteria 4% 62% Xanthomonadaceae 3% Verrucomicrobiaceae 3% Cytophagaceae 3% Micrococcaceae 3% 2% 2% Hyphomicrobiaceae Microbacteriaceae All others Figure 6: Ten most abundant bacterial OTUs by abundance over 24 samples taken throughout the CNC Algaewheel
primary treatment tank. (unclass bacteria represents a single OTU that was not taxonomically distinguishable).
Discussion
Coupling the advantages of fixed film bacterial and algal biofilms into a hybridized treatment
system offers the benefits of traditional treatment with synergy of algal oxygen production and
leverages the symbiosis of a diverse aquatic ecosystem for the treatment of wastewater. The
advantages were demonstrated in the system’s ability to process highly variable waste
concentrations on a daily and seasonal basis.
The system is capable of treatment because of the positive oxygen balance generated through the
photosynthetic production of oxygen discussion. Each wheel with in the system is capable of
generating 32 grams of oxygen on the daily basis assuming 16 hours of daylight based on the
estimates of Epping 3 and Bernstein 4. The total oxygen production for the CNC system is 2880 g
day-1 with an oxygen surplus of 1057 g day-1 before considering the traditional factors of oxygen
8 transfer into biofilms, primarily gas to liquid diffusion and the use of mechanical aeration or air
pumps. The photosynthetic generated oxygen is also produced within the biofilm. Oxygen is
diffusing out of the biofilm when photosynthesis is occurring as opposed to diffusing through the
boundary layer of the film effectively eliminating the oxygen limitation within the film while
lighted. When not in the presence of light the CNC system becomes dependent upon the same
gas transfer mechanisms as other fixed film systems such as rotating biological contactors. This
does not reduce the effectiveness of the CNC system since the flow of the system is primarily
during the day when the park is open, reducing during the night.
Because of the photosynthetic oxygen there are two primary benefits to the system 1) the
aeration to maintain high bulk water dissolved oxygen levels is not necessary, 2) total energy
input to supply more oxygen or to force the oxygen through fine diffusers to increase oxygen
transfer rates is not needed. Both of these traditional costs are not needed since the algal
produced oxygen not only increases the total mass of oxygen in the system but the diffusion
gradient resulting from photosynthesis results in oxygen moving out of the biofilm into the bulk
water instead of generating an oxygen deficit which is typical in fixed film systems.
The system is fundamentally more diverse than traditional bacterial systems through the
incorporation of photosynthetic microbes. There is a symbiosis between the photosynthetic and
heterotrophic portions of the community. The Photosynthetic organisms produce oxygen whilst
the heterotrophs produce carbon dioxide as waste. Oxygen and carbon dioxide are then inputs
for the other group. Autotrophic nitrifiers also generate H+ during the oxidation of ammonia
reducing pH while photosynthesis consumes CO2 increasing pH. It has also been shown that
bacterial enzymatic activity increases in the presence of algae 2, 5, 14. It has also been shown that
heterotrophic micro-organisms can utilize various algal exudates as an additional organic carbon
source 14, 15. This reinforces the concept of improved treatment through increased biodiversity
and niche partitioning which has been described in natural waterways 5, 14. As the community is
diversified the micro-organisms present although all utilizing dissolved organic carbon (CBOD),
may have different limiting nutrients across the CBOD utilizers or may exhibit higher CBOD
utilization based upon the structure of the carbon 14, 16.
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