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Biosolids mineralization in an anaerobic–aerobic combinedreactor system

Prathap Parameswaran1, Paul R. Anderson�

Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, IL, USA

a r t i c l e i n f o

Article history:

Received 31 October 2006

Received in revised form

15 February 2007

Accepted 26 February 2007

Available online 27 April 2007

Keywords:

Biosolids

Mineralization

Aerobic

Anaerobic

Reactor

nt matter & 2007 Elsevie.2007.02.048

thor. Tel.: +1 312 567 3531;andersonp@iit.edu (P.R. Ass: Center for Environm85287, USA.

a b s t r a c t

In this paper, we describe a biosolids mineralization process that could address some

concerns about biosolids management. Solids retention in a combined anaerobic/aerobic

reactor system promotes biosolids mineralization. Solids retained in the reactor were

subjected to both anaerobic and aerobic degradation in two different zones in the reactor.

After 267 d of operation, 75% of the solids that entered the reactors were mineralized, 62%

of the total nitrogen (TN) was transformed, 51% of the phosphorus in the reactors was

precipitated, and 39% of the solids dissolved and appeared in the effluent. Accumulation of

solids in the reactors did not have an adverse effect on reactor performance. Evidence of

biosolids degradation included a decrease in the VSS/TSS ratio, an increase in temperature,

loss of nitrogen, decrease in COD, and an increase in TDS.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Biosolids are the (primarily organic) residual materials

generated from wastewater treatment processes. The United

States Environmental Protection Agency (USEPA, 1999) esti-

mated that 6.9�106 tons of biosolids were generated in 1998.

The majority of that material (60%) was managed through

beneficial applications such as land application and compost-

ing. The remaining 40% was discarded through other means,

such as in landfills or by incineration.

Prior to use or disposal biosolids characteristics are usually

modified through stabilization and dewatering processes.

Stabilization processes reduce the volatile solids content,

odor, and amount of pathogens. Dewatering is an important

aspect of traditional management because raw biosolids are

typically more than 90% water, which complicates and adds

to the expense of biosolids management.

Standards for the use or disposal of biosolids are covered in

the Code of Federal Regulations Title 40, Part 503, published in

r Ltd. All rights reserved.

fax: +1 312 567 8874.nderson).ental Biotechnology, The

the Federal Register in 1993. Biosolids management, however,

is controversial. There has been public opposition based on

concerns about odors, disease, and contaminants associated

with biosolids. For example, in 2001 the National Whistle-

blower Center expressed concerns to the Office of the

Inspector General about USEPA position on biosolids. In

response, the inspector general conducted an assessment of

their program (USEPA, 2002). The executive summary of that

report notes that USEPA believes the biosolids program is

‘‘low-risk and low-priority’’. In contrast, about that same time,

the National Research Council Board on Environmental

Studies and Toxicology reviewed recent progress in risk

assessment and recommended that uncertainties and data

gaps in biosolids management should be revisited (NRC,

2002). Biosolids management should become a growing

concern as the mass of biosolids increases. The USEPA

(1999) estimated that total biosolids production in the US

would increase to 7.6�106 tons in 2005 and to 8.2�106 tons in

2010. Biosolids management is already expensive; Wei et al.

Biodesign Institute, Arizona State University, 1001 S McAllister

ARTICLE IN PRESS

Fig. 1 – Process schematic showing a single reactor with

lower anaerobic and upper aerobic zones.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 2 7 3 9 – 2 7 4 72740

(2003) estimated that treatment and disposal of biosolids

accounted for up to 60% of the total operating costs of

wastewater treatment plant operations.

One intriguing option for biosolids management is com-

plete mineralization, an idea that can be traced back at least

to the work of Porges et al. (1953) who operated an activated

sludge process without solids wastage. They theorized that

the process would reach a steady-state condition where the

rate of biosolids production was balanced by the rate of

biosolids auto-oxidation. Subsequent support for the total

oxidation theory came from studies by Horton (1954) and

Forney and Kountz (1958). There was, however, growing

criticism of the concept of total oxidation (Symons and

McKinney, 1958; Stack and Conway, 1959; Busch and Myrick,

1960). Those authors suggested that an inert organic fraction

would continually build up in the sludge. In fact, tests

confirmed that the inert constituents of the solids were

mainly extra cellular polysaccharides (Washington and

Symons, 1962). Washington and Rao (1964) also reported that

there were long-term cyclic patterns in the solids concentra-

tion, possibly attributed to the cyclic adaptation of organisms

over several years of accumulation.

Gaudy and co-workers conducted a series of experiments to

determine whether accumulation of solids would lead to

ultimate failure of a reactor operated with no solids loss. In

contrast with the steady-state reported by Porges et al. (1953),

Gaudy et al. (1970a) observed cycles of increasing and

decreasing solids concentration. The authors suggested that

these cycles in solids concentration occurred because in a

total solids retention process there could be shifts in bacterial

and predator populations. After 1.5 years of bench scale

reactor operation, Gaudy et al. (1970b) concluded that the

total oxidation concept was theoretically sound. To increase

the rate of biosolids mineralization, they explored a side-

stream hydrolysis process. In their system, biosolids were

retained, subjected to an acid hydrolysis step, and returned to

the reactor. Yang and Gaudy (1974) examined side-stream

hydrolysis stability when subjected to shock loading and

reported that the process was stable.

To test the process performance under the presence of a

large quantity of inorganic solids, Gaudy et al. (1976)

conducted pilot studies using trickling filter sludge with

solids ranging from 40,000 to 80,000 mg/L; the ash content

ranged from 30% to 44%. They reported that the process

was successful, but cautioned that the significant increase in

total dissolved solids (TDS) concentration in the effluent

could be a disadvantage. Parameswaran (2005) provides

a more complete review of recent work on biosolids

minimization.

In this study, we examine a biosolids treatment process

based on a bench-scale, Sheaffer modular reclamation and

reuse system (SMRRS) for biosolids mineralization. Sheaffer

International L.L.C. has experience with over 60 wastewater

management projects dealing with municipal, industrial, and

agricultural wastewaters (Sheaffer International, 2005). An

SMRRS typically consists of three basins (reactors) in series.

Macerated influent wastewater is introduced near the bottom

of the first reactor, below static tube aerators. This same

configuration applies to all basins so that each reactor has a

lower anaerobic zone and an upper aerobic zone. The

theoretical hydraulic residence times (HRT) in each of the

first two reactors is about 21 d. The third basin, which has

minimal aeration, is primarily used as a storage reservoir.

Effluent from these systems is typically used to irrigate

grasslands or crops. Solids removal is not part of the SMRRS

process design and anecdotal evidence from more than 25

years of experience with various SMRRS installations sug-

gests that there is very little solids accumulation in the

basins. The purpose of this study was to evaluate the fate of

biosolids in a model SMRRS and assess the likelihood of an

SMRRS as a biosolids mineralization process. Specific ques-

tions we hoped to address were:

�

Do biosolids accumulate in the reactor?

�

What is the fate of inorganic solids?

�

Does solids retention have any adverse effect on reactor

performance?

�

What were the fates of nitrogen and phosphorus in the

process?

2. Materials and methods

All tests were conducted at the 12 MGD (45,400 m3/d) Greene

Valley wastewater treatment facility in Woodridge, IL. This

facility has bar screens, but there is no primary settling prior

to the aeration basins. Based on discussions with plant

personnel, the SRT is typically 10–12 d, depending on condi-

tions in the influent and the secondary clarifier. There are no

special process modifications for phosphorus removal at this

facility. Waste activated sludge (WAS) for our tests came from

the secondary clarifier underflow.

Two test reactors were assembled from transparent

Plexiglass cylinders, approximately 30 cm in diameter with a

height of 122 cm for a total volume of about 86 L (22.7 gallons)

(Fig. 1). The bottom of each cylinder was attached to a square

(30.5 cm or 1 ft per side) Plexiglass plate; these parts were

connected and sealed with solvent. Each reactor had two

ARTICLE IN PRESS

WAT E R R E S E A R C H 41 (2007) 2739– 2747 2741

1.25 cm (0.5 in) diameter fittings with valves, one located

about 7.6 cm (3 in) and the other 91 cm (36 in) above the

bottom of the reactor. A fritted aeration stone was suspended

30 cm above the bottom of the reactor on an aluminum

bracket. Each aeration stone was connected to a two horse-

power electric air compressor with a seven gallon storage

tank capable of delivering 142 L/min at 2.76�105 Pa (5 SCFM

at 40 psi). Air flow to each reactor was monitored and

controlled (142 L/h or 5 SCFH) by an in-line flow gauge.

This aeration stone separated the reactor into an unmixed

lower anaerobic zone and a mixed upper aerobic zone. Based

on the relative volumes, theoretical hydraulic residence times

for the anaerobic and aerobic zones were 6.7 d and 13.3 d,

respectively.

Fresh WAS (well-mixed) was obtained through a sample

port connected to the secondary clarifier underflow. To help

ensure a representative sample, the first flow from the port

was wasted prior to sample collection. On the first day of

testing, the reactors were filled with WAS and the top of each

reactor was covered lightly with plastic wrap to minimize

evaporation. On subsequent days the reactors were operated

in a semi-batch mode with a brief combined feeding (2.8 L d�1)

and monitoring process. To minimize mixing in the anaerobic

zone during feeding, pumping was controlled by a peristaltic

pump at a rate between 40 and 50 mL/min so that the feeding

period lasted about 1 h. Effluent samples were collected from

the effluent removal port (Fig. 1) during this 1 h feeding

period.

Results reported here cover the first 267 d of the study.

During the first 39 d of testing, the air supply operated

constantly. In that mode, effluent from the reactor came

directly from the well-mixed aerobic zone and had a relatively

high suspended solids concentration. Because one of the

objectives of this study was to minimize the loss of sus-

pended solids from the reactor, from day 40 onward before

starting the feeding process we turned off the aeration system

to allow solids to settle. Once a clear interface in the aerobic

zone was visible the feed pump was started and the effluent

sample was collected. The aeration system was turned on

again at the end of feeding.

Solids were also occasionally transported from the anaero-

bic zone into the aerobic zone by upwelling. These events

occurred when gases produced during anaerobic degradation

coalesced to form bubbles that became trapped in agglomer-

ated particles. Rising bubbles with attached particles could be

seen through the sides of the reactors.

Sampling and analyses were performed on a daily basis to

assess reactor performance. Physical analyses included

measurement of total suspended solids (TSS), volatile sus-

pended solids (VSS) and total dissolved solids (TDS), accord-

ing to standard methods (Clesceri et al., 1995). Temperature,

pH, dissolved oxygen (DO) were measured directly using a

multi probe system (YSIs 556 MPS). Chemical analyses

included the determination of total and soluble chemical

oxygen demand (COD), total nitrogen (TN), total phosphorus

(TP), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N),

and soluble reactive phosphate (PO43�) using a HACH DR 2500

spectrophotometer.

Although it was not part of the original study, we began to

measure the solids concentration throughout the mixed

reactor to provide information for a material balance

calculation. For this measurement, we inserted a rigid

1.25 cm (0.5 in) diameter tube to the bottom of the reactor,

sealed the top, and then slowly removed the tube from the

reactor. This procedure was repeated four times at equal

intervals around the diameter of the reactor, a single

composite sample was prepared for each reactor, and the

samples were analyzed to determine the TSS and VSS. In

these and other solids measurements the mass of solids

removed from the reactor was calculated and the material

balance expressions were corrected for the loss of solids.

Chemical equilibrium calculations, conducted to assess the

potential for phosphorus precipitation in the reactors, were

carried out using the equilibrium software package Visual

MINTEQ (Gustafsson, 2000).

The quality assurance/quality control (QA/QC) program

included blank and replicate measurements to assess the

accuracy of the measurements. Results, expressed as relative

standard deviations (standard deviation divided by the mean)

were typically less than 10%, which is the maximum

recommended by Clesceri et al. (1995) and HACH (2003).

3. Results

Solids retention depends on the operation mode (Fig. 2). With

the aeration system turned off during sample collection

(t440 d) the effluent total solids concentration is typically

less than 2% of the influent value. Similar patterns appear for

TN (not shown) and TP (Fig. 3). When samples are collected

during quiescent conditions the effluent TN and TP concen-

trations are about 8% and 27% of their respective influent

concentrations.

Average TDS concentrations in the effluent increase about

20% relative to the influent TDS concentration (Fig. 4). Other

operating parameters (data not shown) indicate the ratio of

soluble-to-total COD in the influent and effluent remains

relatively constant at 0.01 and 0.61, respectively, and the pH

values in the aerobic zone average 7.1 (standard deviation ¼

0.9) with no obvious trend throughout the test. Dissolved

oxygen concentrations in the aerobic zone range consider-

ably, from less than 1 mg/L to more than 7 mg/L. Average

influent and effluent concentrations are 0.9 and 23.1 mg/L,

respectively, for NO3-N, and 20.6 and 3.0 mg/L, respectively,

for NH3-N.

Results for most of the water quality parameters are

substantially less relative to their average influent concentra-

tions, but this decrease in concentration could be due to

mineralization or due to accumulation within the reactor. To

distinguish between these two options, we consider mass

balance equations.

The mass balance expression is

accumulation within the reactor ¼ influent� effluent

� net effect of mineralization:

The last term on the right-hand side could describe more

than one reaction. For example, fixed solids could increase as

a result of precipitation within the reactor or they could

decrease as a result of dissolution within the reactor. In

general, these reactions could occur in parallel and the

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0

2000

4000

6000

8000

10000

12000

0 50 100 150 200 250 300

Duration of test (days)

TS

S (

mg/L

)

Influent

Effluent R1

Effluent R2

Fig. 2 – Influent and effluent total suspended solids concentration as a function of time.

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250 300

Duration of test (days)

Tota

l P

(m

g/L

)

Influent

Effluent R1

Effluent R2

Fig. 3 – Influent and effluent total phosphorus concentrations as a function of time.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 2 7 3 9 – 2 7 4 72742

magnitudes of these reactions could change over time as

conditions within the reactor change.

The influent term of the material balance includes the

initial loading and the daily feed to the reactor. The effluent

term includes mass lost from the system with samples taken

from the reactors (effluent or composite samples). We can

determine the net effect of mineralization by difference if we

can quantify accumulation within the reactor.

There are two ways to estimate solids accumulation in the

reactor. One method relies on data from the mixed aerated

zone. Unfortunately, there are no data for 40pt (d) p105 and

data from the other times do not include solids accumulation

in the anaerobic zone. Nevertheless, the available data

suggest that TSS accumulation in the reactors ranged from

40.9 to 44.2 mg/L d (Fig. 5). The second method relies on

weekly composite samples (including the anaerobic zone);

these data (not shown) exist for tX137 d. Using this information

and the initial solids concentration we estimate that TSS

accumulation ranged from 38.7 to 43.9 mg/L d.

From this range of interpolated values we calculate a

constant mean mass accumulation rate (Table 1) to use in

material balance equations. To complete mass balance

calculations for TN, we follow the suggestion of Grady and

Lim (1980) and assume the nitrogen fraction in the VSS is

0.125. To complete the mass balance for TP we assume that

phosphorus makes up 0.061 of the VSS.

The resulting material balance for VSS, for example,

indicates that the rate of mineralization increases slowly

during the first 25 d and remained at a relatively constant rate

for the duration of the study (Fig. 6). Around day 40

(coincident with the change in sampling procedure) of the

study, the mass of VSS mineralized exceeds the mass of VSS

leaving in the effluent and around day 70, mineralization is

the dominant fate.

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1 10 100 1000 10000

COD

Total P

Total N

TDS

VSS

TSS

Concentration (mg/L)

2%

27%

8%

121%

1%

2%

Fig. 4 – Average influent (speckled) and effluent (dark) concentrations for major water quality parameters during the test.

Because of the wide range in values, the concentration axis has a log scale. All samples were collected under quiescent

conditions.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 50 100 150 200 250 300

Duration of test (days)

Mix

ed

zo

ne

TS

S (

mg

/L)

Fig. 5 – Measured and interpolated TSS for the mixed effluent in the aerobic zone as a function of time.

Table 1 – Range and mean value of mass accumulationrates for TSS, FSS, and VSS

Parameter Range (mg/L d) Mean (mg/L d)

TSS 39–44 41.5

FSS 19–21 20

VSS 19–26 22.5

WAT E R R E S E A R C H 41 (2007) 2739– 2747 2743

Material balance results for TP indicate that dissolution initially

exceeds precipitation (Fig. 7). Phosphorus precipitation increases

after about 40d, it exceeds accumulation in biomass after about

130d, and it exceeds the mass of TP in the effluent after about

150d. Chemical equilibrium modeling (Visual MINTEQ) suggests

that hydroxyapatite (Ca5(PO4)3OH) precipitates in the reactors.

Struvite (NH4MgPO4) apparently does not precipitate at the

relatively low NH4+ concentrations in the reactors. If all the

phosphorus attributed to precipitation occurs as hydroxyapatite,

the mass would be 231g, suggesting that hydroxyapatite

precipitation accounts for 65% of the total FSS accumulated in

the reactors. In contrast, the material balance for TN (Fig. 8)

indicates that most of the influent TN is not accounted for,

presumably lost from the reactor in the gas phase.

Data from the mixed aeration zone exhibit substantial

noise—probably due to upwelling and settling effects—as can

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TSS R1 = 43.9 t + 6413

TSS R2 = 38.7 t + 6416

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 50 100 150 200 250 300

R1

R2

Reacto

r C

om

posite T

SS

(m

g/L

)

Duration of test (days)

Fig. 6 – Measured and interpolated TSS as a function of time. These estimates are based on composite samples from the

entire reactor volume.

-50

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

Duration of test (days)

Tota

l P

mass (

g)

Influent

Effluent

Precipitated

Biomass

Biomass

Influent

Effluent

Fig. 7 – Material balance results for total P. Influent and effluent results were measured, biomass accumulated was measured

and interpolated, and the precipitated values were determined by difference.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 2 7 3 9 – 2 7 4 72744

be seen in the TSS results (Fig. 5). However, using a moving

average to filter this noise reveals cyclic patterns that can also

be seen in data for other monitored parameters (Fig. 9). These

patterns may provide evidence for the type of cyclic

predator–prey relationships suggested by Gaudy et al. (1970a).

Summary results from the material balance calculations

(Fig. 10) show that most of the influent TP ends up

precipitated within the reactor and most of the TN is lost

(transformed) to the gas phase during mineralization. Nearly

70% of the influent fixed solids leaves the reactor as soluble

effluent, and about 70% of the influent VSS is mineralized.

Other data supporting the idea of mineralization come from

the change in VSS and the change in temperature. The VSS/

TSS ratio decreases from 0.83 in the feed to around 0.65 in the

reactor mixed effluent. The average influent, ambient, and

effluent temperatures are 15.9, 16.6, and 22.0 1C, respectively.

This increase in temperature through the reactors is

consistent with biochemical degradation. For example,

Grady and Lim (1999) suggested that the heat released by

degradation of VSS should be about 1.88�104 kJ/kg

(9�103 BTU/lb). The average VSS degradation rate from

our material balance analyses can be used to estimate the

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0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300

Duration of test (days)

Tota

l N

mass (

g)

Influent

Effluent

Biomass

Not accounted for Influent

Effluent

Biomass

Not accounted for

Fig. 8 – Material balance results for total N. Influent and effluent results were measured, and biomass accumulation was

measured and interpolated. The solid line (determined by difference) represents nitrogen not accounted for in the material

balance, which was presumably transformed to gaseous nitrogen compounds and lost from the reactor.

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300

Duration of test (days)

Solu

ble

phosphate

(as P

O4)

(mg/L

)

Effluent R1 moving average

Effluent R2 moving average

Fig. 9 – Effluent soluble phosphate concentration expressed as a moving average with n ¼ 7.

WAT E R R E S E A R C H 41 (2007) 2739– 2747 2745

rate of heat production:

10:9�gVSS

d

� �� 18;800

Jg

� �¼ 2:05� 105 J

d

� �.

About one-third of that total heat lost from one reactor

can be attributed to the effluent flow from that reactor

each day:

2:8Ld

� �� 1

kgL

� �� 4180

Jkg K

� �� 6:1ðKÞ ¼ 7:1� 104 J

d

� �.

The rest of the heat must be transferred via convection

to the surroundings. Adopting a simple heat transfer model,

we represent the reactor as a uniform, constant tempera-

ture, vertical tube in air, with heat transfer dominated

by natural convection. In this model, the reactor walls

(kE0.2 J/m s K) offer little resistance to heat transfer; most

of the resistance occurs in the air boundary layer surroun-

ding the reactor. Using the Churchill and Chu correlation

we estimate a heat transfer coefficient in this boundary

layer of hE2 J/m2 s K (Lienhard and Lienhard, 2006).

ARTICLE IN PRESS

0.0 0.2 0.4 0.6 0.8 1.0

VSS

FSS

TOTN

TOTP

Effluent Biomass Precipitate Mineralized Transformed

Fig. 10 – Summary material balance showing the fate of total phosphorus, total nitrogen, fixed suspended solids, and volatile

suspended solids in the reactors.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 2 7 3 9 – 2 7 4 72746

Assuming an average temperature gradient of 2.4 1C the

rate of heat loss is 6.7�105 J/d, or about four times the

predicted rate.

Data obtained through 267 d indicate that the reactors were

accumulating solids. We can estimate the solids retention

time from:

SRT ¼mass of suspended solids in reactor

rate of suspended solids mass leaving reactor.

Terms in the numerator and the denominator change over

time, but after 267 d of operation we estimate SRTs of 4045 d

for VSS and 5043 d for FSS.

4. Conclusions

During the long solids retention times, we believe the solids

oscillated between anaerobic and aerobic zones. It is likely

that the different degradation pathways in these two zones

worked to enhance biosolids mineralization. In addition, we

find that:

�

For the duration of this study there was no evidence that

solids accumulation had an adverse effect on reactor

performance.

�

Based on the material balance calculations, accumulated

solids accounted for about 20% of the influent TP, about

35% of the influent TN, and about 30% of the influent fixed

suspended solids.

�

Almost half of the influent TP ended up precipitated

within the reactor and most of the TN was transformed to

the gas phase during mineralization. Approximately 70%

of the influent VSS was mineralized.

Acknowledgments

We appreciate all the support from Sheaffer International,

L.L.C. We would also like to thank personnel at the

Woodridge—Greene Valley wastewater facility for their co-

operation with this study.

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