Compost Product Optimization for Stormwater
Biofiltration Applications
Paper # 1209
Martin Alcala, Jr., Kim D. Jones, and Jianhong Ren
Department of Environmental & Civil Engineering
Texas A&M University-Kingsville
Kingsville, TX 78363
USA
ABSTRACT
Compost material has been proposed for use in bioreactors for environmental restoration
in some areas to remediate contaminated water and soil. Compost material is ideal for
biofiltration because it supports large microbe populations, which may be capable of
metabolizing numerous natural and xenobiotic compounds. The objective of this project
was to develop techniques to produce high quality compost from typical solid waste
materials targeting nitrate removal in stormwater biofiltration applications. Compost
products were produced from different feedstocks and evaluated for their nitrate removal
efficiencies. Six different compost products manufactured from varying feedstock
amounts of wood waste; grass cuttings and biosolids were evaluated using column studies.
The initial compost product quality was evaluated using Total Kjeldahl Nitrogen (TKN)
and Total Organic Carbon (TOC). An initial nitrate concentration of 60 mg/L was used
for evaluation of nitrate removal. The carbon to nitrogen (C/N) ratio was determined
from TOC and TKN measurements. The compost product’s pH and soluble salt content
were also tested. Effluent water from the columns was tested daily to determine the
amount of nitrate removed over time. Total initial and final nitrogen species were tested.
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INTRODUCTION
Nitrate pollution of water is a great concern throughout the globe. Nitrate levels in
surface waters are frequently elevated by stormwater runoff. Biological, chemical, and
physical methods have been used to lower nitrate concentration levels. Biological
removal by microbes is considered very economical and environmentally friendly.1
Denitrification, microbial removal of nitrate, has been applied to remove nitrates from
wastewater to protect watersheds from eutrophication and to treat agricultural runoff.2
Bioretention is a water quality and quantity control process that utilizes soil, plants, and
microbes to remove pollutants. It has been used in developed areas to treat stormwater
runoff.3 However, nitrate can not be removed by the soil because of its lack of affinity.
Since nitrate is degraded slowly through denitrification processes, excess nitrogen builds
up in biorentention systems. Therefore, it is desirable to promote the conversion of
accumulated nitrogen species to nitrogen gas. A biofilter column study conducted by
Hunho et al. in 2003 found that different nitrate removal efficiencies were observed by
changing the electron donor and carbon source in the column.3
Compost material has been used in bioreactors for environmental restoration in some
areas to remediate contaminated water and soil. Compost biofiltration can remove many
contaminants especially organic chemicals in water. A study conducted by Bohnke and
Eitner in 1983 demonstrated a removal efficiency of 70% for chlorobenzene, 80% for
toluene, and almost 100% for terpene from water using compost biofilter blends of
municipal solid wastes (MSW) and biosolids. Removals for benzene, dimethylsulfide,
and tetrachloroethylene exceeded 85%.4 Compost material is ideal for biofiltration
because it supports large microbe populations including bacteria, fungi, actinomycetes
and insects. These microbes are capable of metabolizing numerous natural and
xenobiotic compounds.5 Compost provides essential nutrients such as carbon and
surfaces to the microbes. The moisture content of the compost also has a significant
effect on the microbial activity. Microbes can metabolize, utilize or oxidize organic
compounds such as nitrates.5
2
Research done by Jakobsen found a minimal leaching of ammonium and potassium and
reduced leaching of nitrate by using compost on plants. 6 In addition, the use of
municipal organic waste (MOW) compost with sound irrigation practices has shown that
nitrate discharge can be controlled and reduced in agricultural runoff.7 However, other
research has shown that while compost may reduce nitrogen leaching, it might increase
nitrate leaching if used for extended periods of time.8 Research conducted by Insam and
Merschak found that uncomposted sewage sludge products applied to forest soil cores
resulted in evaluated measured amounts of nitrates in the soil water and in the leachate.9
It is possible to have nitrate leachate during composting, but the final compost product
may be very useful for nitrate reduction. The application of compost increases the
activity of heterotrophs, which might contribute to the nitrate reduction in agricultural
runoff. Since the heterotrophs also supply NH4 to soil nitrifiers, the addition of compost
and the effect that it has on ammonification, nitrification, and the assimilation of nitrogen
by microorganisms has not been well characterized.10
The City of Brownsville, in an effort to minimize solid waste at the Brownsville
Municipal Landfill, has been operating a composting facility for several years. The
compost is primarily made up of brush, lawn waste, and zoo animal waste. Optimal
composting requires moisture content of 40-60% by weight; however the Rio Grande
Valley is a semi-arid region and water conservation and reuse is an important concern.
Thus, we propose to use the biosolid sludge produced from the wastewater treatment
plants operated by the Brownsville Public Utility Board (PUB) as a potential source of
moisture for compost. By combining the biosolids from the Brownsville PUB with the
compost feedstocks from the landfill, initial moisture content targets can be met. In
addition, a special purpose of using compost products for denitrification could possibly
be achieved through manipulation of the feedstocks, including biosolids, during the
compost process. The denitrification potential of the final compost product produced by
co-composting the biosolids and the lawn and garden waste could develop a potential use
and market for the products that may help other municipalities in the Rio Grande Valley.
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COMPOST METHODOLOGY
A Columbus Instruments Oxymax-C composting reactor shown in Figure 1 is being used
to develop compost products using different feedstocks.
The reactor has specially designed air-flow and temperature controllers to control
temperature, aeration, and moisture for creating a stable environment for composting.
For example, temperature is controlled by regulated air flow as shown in Figure 1. The
isothermal composting reactor is designed to accelerate the decay process. The compost
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reactor has a total volume of 38 L. A schematic graph of the reactor is presented in
Figure 2.11
Compost Feedstock Blending
The following table shows the blends that are being used in this study:
Table 1. Compost Feedstocks.
Compost
Blend
1(trial)
2(control)
3
4
5
6
Wood Chips (% by Grass Clippings (% by
volume)
volume)
Biosolids
17
78
0
50
50
0
70
30
0
90
10
0
10
90
0
50*
30*
20*
*depending on biosolids metals analysis
Wood chips and grass clippings have been used in equal amounts in compost blend 2,
which was used as the control. The wood chips are obtained from the City of Brownville
Landfill (CBLF). Grass clippings are collected from Kingsville, TX. Different moisture
levels were obtained by changing the amount of grass and wood. The materials are
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composted for 21 days. Wood chips and grass clippings both have an average specific
gravity of 400 lb/yd3 through which the total starting weight was calculated.12
Test Parameters
The compost process and compost product stability is characterized and evaluated using
several parameters including Total Organic Carbon (TOC), Total Kjeldahl Nitrogen
(TKN), moisture, Respirometry, and pH.
TOC and TKN
The compost starting mix and final products are being tested for Total Kjeldahl Nitrogen
(TKN) and Total Organic Carbon (TOC). Digestion is used to break down the material to
test for TOC and TKN. Digestion is achieved through the Hach Digesdahl system. The
Digesdahl Digestion Apparatus can digest many types of samples such as sludges, food,
and soils. Sulfuric acid and hydrogen peroxide are used to oxidize the material. The C/N
ratio was determined from TOC and TKN measurements.13
Moisture
Final composted product, wood chip, and grass percent moisture are determined through
dry mass measurements. This is done gravimetrically by drying the sample at 110C for
24 hours. The sample is dried using a single wall gravity convection oven manufactured
by Blue M model SW-17TA. The weights are taken using a Fisher Scientific scale model
A-160.
Respirometry
Respirometry was monitored by observing the O2 consumption and CO2 production rates.
The Oxymax-C system has an electrochemical oxygen sensor with a range of 19.3% to
21.5%. The Oxymax-C carbon dioxide sensor uses single beam infrared light to
determine gas levels and has a range of gas levels of 0% to 10%. The O2 and CO2
sensors were calibrated using a gas mixture of 5% CO2, 20.6% O2 balanced in N2
purchased from A-L Compressed Gases.
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pH
The compost product’s pH is determined by making a compost extract. The extract is
made by mixing a 5g dried sample with 25mL of distilled water. The pH is then
measured using a HACH pH meter model EC 40 pH/ISE meter.
Compost Column Studies
Four columns have been made of clear PVC pipes with the following design
parameters:
Table 2. Compost Column Specifications for Biofiltration.
Parameter
Diameter of Columns
Length of Columns
Inlet Flow Rate
Nitrate Concentration (mg/L)
Value
2.0 in
10.0 in (x2) and 20.0 in (x2)
0.25 LPM
60
Two columns have a column length of 10 in, and the other two columns have a column
length of 20 in. This was designed to observe the effects of different retention times on
composting. The 10 in and 20 in columns have a volume of 0.5 and 1.0 L, respectively.
The 0.5 L columns were filled with 120 g of composted material while the 1.0 L columns
were filled with 238 g of composted material. The columns were fitted with a ¾ in valve.
A grill made from green mesh material was put between the PVC pipe and the valve.
This was to ensure that no compost material was lost when water flowed through the
columns.
An initial sodium nitrate concentration of 60 mg/L was used to flow through each column
loaded with compost material. Twelve 250 mL effluent water samples were taken at 30
min intervals from each of the columns. Total initial and final Kjeldahl Nitrogen (TKN),
NO3- (nitrate), NO2- (nitrite), pH, and conductivity were tested. TKN, NO3-, and NO2- are
determined using the Hach Laboratory Spectrophotometer (Model DR/2500). TKN is
determined using Nessler Method which is suitable for wastewater and sludge. Nitrate
and nitrite are determined using the Hach Method 8039 and Method 8153, respectively.14
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Preliminary Results
The first compost run (Run 1) consisted of 17 % wood chips, 78 % grass clipping, and
5% tap water. The composting process became anaerobic after two weeks and was
terminated. Possible reasons for the anaerobic conditions include too much moisture and
pockets of anaerobic activity forming in the chamber since a large amount of water was
collected at the bottom of the chamber. This might be due to the fact that the grass
clippings used for starting materials had more water than was initially assumed. In
addition, the materials in the compost chamber were tightly packed which could have
contributed to the formation of anaerobic regions. The material was approaching stability
from an oxygen consumption frame of reference within 30 days until the anaerobic
conditions began to dominate the system (Figure 3).
Figure 3: Respirometry Compost Run 1
3500
Oxygen Comsumption (VO2)
O2 Consumption/ CO2 Production
(mg/kg/hr)
3000
Carbon Dioxide Production
(VCO2)
2500
2000
1500
1000
500
0
0
5
10
15
20
25
30
35
40
45
50
-500
Time (hr)
A second experimental run (Run 2) was conducted that consisted of a mixture of 50 %
wood chips to 50% grass by volume. Before the compost reactor was loaded the starting
materials were processed. The wood chips from the Brownsville Municipal Landfill
were chipped and shredded to a smaller and more uniform size. The material was then
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sieved to remove any powder. This material was composted for a total of 140 days to
ensure maximum stability. The respirometry indicates that the compost became
biologically active at 40 days from the start of composting while activity diminished after
the 80-day mark (Figures 4a and 4b).
Figure 4a: Respirometry Compost Run 2Oxygen Consumption
O2 Consumption (mg/kg/hr)
700
600
500
400
300
200
100
0
0
20
40
60
80
Time (days)
9
100
120
140
Figure 4b: Respirometry Compost Run 2Carbon Dioxide Production
80
CO2 Production (mg/kg/hr)
70
60
50
40
30
20
10
0
0
20
40
60
80
100
120
140
Time (days)
A third experimental run (Run 3) was conducted that consisted of a mixture of 90 %
wood chips to 10% grass by volume. The material was composted for 21 days. A fourth
experimental run corresponding to Table 1 was conducted that consisted of 70% wood
chips and 30% grass. This material was also composted in the reactor for 21 days. The
produced materials are currently being processed for the nitrate removal column tests.
CONCLUSIONS

Compost products are being developed and tested from various common
feedstocks from South Texas, including biosolids, for potential application in
biofiltration of stormwater.

For these feedstocks the compost product appears to reach complete stability
within approximately 90 days of composting time.

In order to develop and test a more active product, composting times of 21 days
are being utilized to produce a material with a more active microbial consortium,
which may include nitrifying bacteria.
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355-67.
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