CET-vol95

DOI: 10.3303/CET2295033

Paper Received: 23 March 2022; Revised: 15 June 2022; Accepted: 19 May 2022
Please cite this article as: Barczak R.J., Fisher R.M., Stuetz R.M., 2022, Importance of Musty Odour Character in the Emissions from
Anaerobically Stabilized Dewatered Biosolids. Towards the Identification of Unknown Odour Components, Chemical Engineering Transactions,
95, 193-198 DOI:10.3303/CET2295033

CHEMICAL ENGINEERING TRANSACTIONS

VOL. 95, 2022

A publication of

The Italian Association
of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Selena Sironi, Laura Capelli

Importance of Musty Odour Character in the Emissions from

Anaerobically Stabilized Dewatered Biosolids. Towards the

Identification of Unknown Odour Components.

Radoslaw J Barczak a,b,*, Ruth M. Fisher b, Richard M. Stuetz b

a Faculty of Chemistry, University of Warsaw, 1 Pasteura Street, 02-093 Warsaw, Poland
b UNSW Australia, School of Civil and Environmental Engineering, Sydney, Australia

rbarczak@chem.uw.edu.pl

Odours from stabilized anaerobically digested wastewater sludge – biosolids can cause local community

odorous impact. Contributions from some volatile organic compounds (VOCs) to nuisance odours are not well

understood. The analytical identification of compounds with low odour detection thresholds (ODTs) at low

concentrations, are challenging. The knowledge about the odour composition from biosolids emission is still

limited. In this study thirty-six biosolids samples were taken after anaerobic stabilisation and dewatering at a

wastewater treatment plant in Sydney, Australia to investigate the musty type of odorous events during gas

chromatography (GC) coupled with mass spectrometer detector (MSD) and an olfactory detection port (ODP)

analysis. Biosolid cake samples were stored under aerobic conditions in loosely covered trays for a period of

35 days. Emissions were collected onto Tenax TA sorbent tubes using a U.S. EPA flux hood method at storage

days 1, 3, 7, 10, 14, 21 and 35. GC-MS/ODP was used to identify musty type odorous events in the biosolids

emissions as potential chemical compounds, however without its chemical detection via MSD. Measured odour

intensities, classified on a scale from 1 to 4, and odour characters were specified by three ODP assessors. Two

musty type odorous events were identified in all biosolid cake emissions. The measured odour intensities of the

musty type odorant(s) did not significantly alter as the biosolids were aged, however varied between biosolids

cakes. Due to its odour intensity, similarity with biosolids odours, frequency of detection and its likely low ODT

two musty odorous events should be considered as a potential odorant(-s) of concern in biosolids emissions.

1. Introduction

Odours are a common occurrence at and around wastewater treatment plants (WWTP), biosolids processing

facilities and biosolids application locations (Hayes et al., 2014; Fisher et al., 2018; Bylinski et al., 2019). In order

to improve strategies for minimising odour impact on local communities (Hayes et al., 2017), it is necessary to

identify odorants and their fate during the biosolids processing, storage and land application. In the past, in

many developed countries biosolids were commonly disposed by incineration, ocean dumping, or to landfills

(Currie et al., 2003). More recently, following the waste hierarchy beneficial reuse of biosolids to agricultural

land are encouraged due to their nutrient content as fertilizers (Collivignarelli et al., 2019). In most countries,

the quality of biosolids applied to land is regulated by environmental legislation but its odorous impact has limited

regulations. Odour impact is one of the key reasons invoking public concern and complaints during land

applications (Bob et al., 2006). The formation of odours during processing, storage, transportation and land

application is still an area of research. Odour production in biosolids is a complex process influenced by several

factors including, but not limited to, process variables within anaerobic digestion and dewatering processes, as

well as the relationships between odour perception and the concentration of odorants (Adams, 2004).

Odours from biosolid emissions are a complicated matrix, containing well known odorants such as volatile sulfur

compounds as well as a variety of volatile organic compounds (VOCs) (Fisher et al., 2017). High concentrations

of odorants such as sulfur and nitrogen compounds, volatile fatty acids, aldehydes, ketones and indole and

skatole (de Luca et al., 1996; Rosenfeld and Suffet, 2004) have been detected in emissions from biosolids and

are known malodours. Many of the odorous compounds are a result of organic matter degradation, generating

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sulfidic and putrid/rancid type odours associated with biosolids (Suffet et al., 2009; Agus et al., 2012). Other

odour descriptors associated with sludge digestion are putrescent, greasy, and cabbage-like odors (Agus et al.,

2012), while the resultant biosolids may have moldy, musty, fecal and fishy odour characters or qualities (Suffet

et al., 2009). Apart from the well-known odorants associated with organic matter degradation, such as sulfur

compounds and ammonia, the participation of other VOCs in the overall odour matrix has not been well

characterised. Such compounds can contribute to the overall odour properties of emissions, as even low impact

odorants that are generally considered unimportant in odour perception can have great significance in total

odour perception (Ryan et al., 2008, Barczak et al. 2022). It is therefore essential to identify and quantify the

trace VOCs, as these compounds can contribute to the overall odour (Zhou et al., 2016).

Currently, there is a knowledge gap regarding the odorants composition of biosolid odours contributing to

earthy/musty/moldy type characters, which Suffet et al. (2009) describes as a one of the 11 dominant odour

categories from biosolids emissions. Fisher et al. (2018) included musty/earthy odour type in biosolids

processing odour wheel. Musty and moldy type odours are assigned categories in the odorous descriptors

related with municipal management facilities: odour wheels dedicated to wastewater, compost, landfill,

wastewater collection network (Suffet et al., 2004; Rosenfeld et al., 2007; Suffet and Rosenfeld, 2007;

Decottignies et al., 2013; Curren et al., 2016).

This study will detail the identification of odorous signals, responsible for musty type odour characters in

biosolids emissions using a combination of analytical and sensorial approaches.

2. Methodology

2.1 Wastewater biosolids process description

Biosolids samples were collected from a mechanical biological WWTP catering for an equivalent population

(EP) of 200 000 located in Sydney, Australia. The biosolids processing included a few steps: primary and waste

activated sludge thickening, 1 or 2 days of retention in an acidified feed well and 22 days of retention in primary

anaerobic gas mixed mesophilic digesters. Following anaerobic stabilisation, the sludge was dewatered using

two high speed centrifuges with the biosolids being conveyed to a storage bay inside a shed.

2.2 Sampling and sample analysis

The 36 dewatered biosolids cakes with the approximate volume of 10L were sampled from the WWTP just after

dewatering. The samples were transported and spread out in a tray with a dimension 0.6 x 0.4 x 0.2 m3. Emission

samples were captured on day 1, 3, 7, 10, 14, 21 and 35 as the biosolids were stored in ambient conditions in

the trays loosely sealed with a lid. In total 218 emission samples were captured using US EPA dynamic flux

hoods operated according to AS/NZS 142 4323.4:2009. High purity nitrogen gas, at 5 L/min, was used to purge

the hood for 30 minutes before the emission samples were collected. Emission samples were collected at a

constant flow rate (100 mL/min for 10 min) using a calibrated SKC sampling pump (SKC Inc., USA) onto Tenax

TA sorbent tubes in triplicate. Each sorbent tube was connected to an additional tube in series in case of

breakthrough from the first tube.

Sample analysis was performed using thermal desorption with a gas chromatography combined with a mass

spectrometry and an olfactory detection port (ODP). A DB-VRX 150 30m×0.25mm×1.4μm column, was utilised

in the gas chromatograph for compound separation. The sorbent tubes were heated at 275°C for 8 min while

high purity helium was passed through the sorbent tubes at the flow rate of 50 mL/min to desorb VOCs and pre-

concentrated them onto a cold trap by a Peltier cooler. Following the sample transfer, the cold trap was heated

to 300°C at 50°C/min heating rate to desorb the VOCs from the cold trap and then injected into the

chromatographic column. A split flow of 20.7 mL/min was applied during tube and trap desorption to prevent

column overloading. The GC column temperature was initially held at 50°C for 2 mins, then raised at a rate of

15°C/min to 200°C, and then held for 5 minutes. The eluent from the gas chromatograph was split between a

mass spectrometer and an ODP. For an assessor to effectively sniff at the port, the split ratio between the MSD

and ODP was set at 2:3 (MSD:ODP). Three ODP assessors, named AS1, AS2, AS3, complying with panelist

requirements according to European Dynamic Olfactometry standard (EN 13725:2003), were used for the

olfactory analysis. Two assessors AS1 and AS2 were trained according to the procedure described elsewhere

(Barczak et al., 2018), whilst AS3 had previous experience with ODP analysis. Each ODP assessor analysed

one of each triplicate sorbent tubes, the first one from the series of two. Intensities of odours detected from the

ODP were recorded using a controller device or by hand with a scale from 1 to 4 (with 1 being the weakest and

4 the strongest odour intensity). Moreover, one assessor extended the scale for additional halves points: 0.5,

1.5, 2.5 and 3.5. ODP assessors used their own odour descriptors based on their own experience as well as

descriptors from published compost and wastewater odour wheels (Suffet & Rosenfeld 2007). An ODP signal

recording an odorous VOC was assumed to be valid if at least one assessor detected an odour signal at a

similar retention time and similar odour character across all samples. The arithmetic means between assessors

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intensity values was calculated and is the basis for further discussion in the paper. Additionally, two assessors

assessed suitable odour descriptors of emission from raw biosolids cake samples prior to emission sampling.

The more detailed methodology was described elsewhere (Barczak et al. 2019).

The sensory research used in this project was conducted with approval of the Human Research Ethics Advisory

Panel ‘H’ Science and Engineering of The University of New South Wales, Sydney, Australia.

3. Results and discussion

Using the GC-MSD/ODP system the two odour events with similar musty odour character were detected. The

first one named M1 has been detected at the median retention time (RT) equal to 11, whilst the second one’s

named M2 median RT was 11.74. The M1 odour was detected 192, 178 and 186 times whilst the M2 140, 63

and 118 times by the assessors AS1, AS2 and AS3 respectively (Table 1). The detailed descriptors used by

assessors are presented in Table 1. The most common descriptors used for AS1 and AS2 were musty and wet

cloth which are close to each other from the odour character perspective. The AS3 has been describing both of

two odorous events as oily and only sporadically as chemical, urine and sulfur. Other descriptors periodically

used at the identical retention time for assessor AS1 was herbaceous, hospital, sweet and few times assessor

was unable to describe odour character; for assessor AS2 were herbaceous, solvent, sweet, hospital and

occasionally astringent, roses, putrid, burning. As both of the odour events were not related to any identified

peak matching the NIST databases it can be only speculated that their odour character change is based on their

concentration or due to the similar retention times of other coeluted chemicals affecting the odour character. In

the other authors work the other musty odorant presented in the biosolids emission 2,4,6-Trichloroanisole (TCA)

was described with other than musty descriptors (Barczak et all. 2019). The authors described the TCA odorous

events such as chemical, hospital, and astringent.

Table 1: Amount of detection and odour descriptors of musty related odorous signals from biosolids

emission

Assesso

Number of

detections

Odour descriptors (percentage% of detection)

M1 M2 M1 M2

AS1 192 140 Musty (89), wet cloth (5), herbaceous

(3), something (3)

Musty (13), wet cloth (46), herbaceous (19),

hospital (14), something (5), sweet (2)

AS2 178 63 Musty (96), wet cloth (1), herbaceous

(1), astringent (1), chemical (1), putrid

(1), roses (1)

Musty (74), wet cloth (5), herbaceous (3),

solvent (3), sweet (9), hospital (2), burning

(1)

AS3 186 118 Oily (98), urine (1), sulfur (1) Oily (96), chemical (4)

The intensities of both musty odorous events over storage time for all 36 biosolids cakes are reported in Figure

1. Both odorous signals were detected in every biosolids cake, however not across the whole storage time. M1

was detected continuously as most of the cakes were aged (no. 5, 11, 14 and 22), however in some cakes (no.

3, 5, 26, 30) it was not detected on the first day of storage, whilst in others (no. 1, 2 and 27) it wasn’t present

until later. In cakes no. 6, 23 to no. 30 and cakes no.33 to 36 it was no longer detectable after day 14 and 35

respectively. For some cases the detection was not observed continuously like in cakes 31 and 32 where there

was not recorded any signal on day 21.

In some cakes (no. 4, 8, 22 and 25) M2 was not detected on the first day of storage, whereas in others (no. 1,

2 and 27) it wasn’t present until later in the cake ageing. In cakes no. 6, 9, 11, 22 to 25, 27, 29 to 31 and no. 2

to 5, 7, 10, 33, 34 and 36 it was no longer detectable after day 14 and 21, respectively. In two cakes no. 26 and

28 the M2 was detected only once, in both cases on day 7. Two detections were observed in cakes no. 24, 25

and 29.

The intensities of these two musty odour character patterns vary between biosolids cakes. The detection across

the whole measurement period is only valid for cake no 14, 18 and 31 but with different intensity trends.

The boxplot of musty odour events odour intensity values in aged biosolids emission averaged between

sampling days over 218 samples are shown at Figure 2. The odour intensity of M1 and M2 detected by

assessors was typically lower than 3 and 2.5, respectively.

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Figure 1: Arithmetic mean of odour intensity values belonging to two odorous signals with musty odour character

obtained by gas chromatography coupled with olfactory detection port between 3 assessor’s in the emissions

from stabilized biosolids cake after anaerobically digestion of wastewater sludge over storage time.

Figure 2: Arithmetic mean of musty odour events odour intensity values obtained by gas chromatography

coupled with olfactory detection port between 3 assessor’s in aged biosolids emission averaged between

sampling days over 218 samples showing mean and confidence intervals (95%) with ranges indicated by

whiskers.

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The presence of musty-like potential odorants in this study agree with Suffet et al. (2009) observation about

odour nuisance emissions during wastewater biosolids composting conducted at two biosolids composting

plants. The authors made sensory assessment and characterisation using Odour Profile Method analysis and

reported that the biosolids compost product might be described by earthy, musty, grassy, woody, hay, manure

odour type descriptors. Moreover, they observed that after about 20 days of composting earthy and musty odour

character dominated and replaced fecal, sewery, and rotten type odours commonly reported in first weeks of

composting. In this study two assessors assessed odour descriptors of raw biosolids cake samples prior to

emission sampling. Following this study and Suffet’s et al. observations it can be concluded that the musty odour

type was most likely masked by fecal, sewery, and rotten odours at the beginning of the biosolid storage.

Barczak et al. (2019) described this observation about stored biosolids, where musty and mouldy odour

characters become more distinct as the biosolids were stored. They observed that odour character changes as

the biosolids are stored from fecal, sewery, and rotten vegetable odour type characters during the first two

measurement days to moldy, musty and wet cloth odour type characters becoming dominant from day 7. The

detected musty, moldy odour types in the work of Barczak et. al. (2019) may be caused, at least partially, by

potential odorant(s) detected in this study due to its presence in emission. In this study the authors were focused

on only two odorous events in the GC-MS/ODP analysis. From the chemical perspective to confirm the

observation of the masking effect of musty odour type by fecal, sewery, and rotten vegetable odour characters

in the raw biosolid emission the analysis of concentrations trends of chemicals responsible for these odours

might be helpful. However, such analysis is not part of this work.One of the attitudes of odours characterization

is based on choosing the key odorants from the matrix. Vitko et al. (2016) narrowed down the large number of

odorants detected at a WWTP to a manageable number of odorants, something they defined as “most

detectable” odorants. Among odorants such as hydrogen sulfide (rotten eggs odour), ammonia, indole and

skatole (fecal odour), they indicated two compounds, named 2-Methyl Isoborneol (MIB) and 2-Isopropyl-3-

Methoxy-pyrazine (IPMP) as potentially most detectable odorants representing the musty odour character. In

this study none of those two odorants were identified. Comparing the retention times of authentic standards of

IPMP and MIB it could be confirmed that odorous events M1 and M2 belong to compounds other than MIB or

IPMP. Taking into consideration the distinctive musty odour character in biosolids emission, the successful

identification of M1 and M2 will allow including it into the potentially key odorants.Detection of odorous events

didn’t correspond to any reasonable signals from MSD. The lack of matches of the particular ions from the area

of chromatograms corresponding to odour events might be caused by the low concentrations of potential

odorants. Concentrations of those potential odorants might be below the detection limit of the instrument used.

4. Conclusions

Very little is known about musty odour character emissions from biosolids. Apart from well-known odorants such

as sulphur and nitrogen compounds, the participation of other VOCs in overall odour mixture should not be

unnoticed. Especially when the odour character hardly corresponds to the identified odorants. The detection of

compounds with low odour threshold that are present at low concentrations is difficult in chemical analysis. The

most popular gas-chromatography devices coupled with common chemical detectors like MS or FID are often

not sensitive enough for detection of odorants present in ppb or ppt concentrations. Instead, approaches using

combined analytical and sensorial methods are useful in identifying the presence of odorants in emission

mixtures. In this study two musty odour character potential odorants were identified in all analysed samples of

dewatered biosolids after the anaerobically stabilization process.

Intensity levels of musty character odorants do not show any increasing or decreasing trend as the biosolids

were aged, whilst both are consistently present in stored biosolid samples. The potential odorants causing the

musty odorous signal seem to be the odorants of importance in biosolids emission due to their low odour

thresholds, frequent detection by ODP assessors and their odour character. Moreover, very limited data exists

on qualitative analysis of musty type odours. Taking into consideration 36 different biosolids cakes at the

different age of storage it could be concluded that due to their intensity and frequency of detection by ODP

assessors two musty odorous events should be recognized in the future and be consider as odorants of concern

in biosolids emission. Thus, the additional effort should be made to identify all odorants responsible for musty

odour character in biosolids emission.

Acknowledgments

This work was supported by a Marie Curie International Outgoing Fellowship under the grant PIOF-GA - 2013-

622523 OdourCOB within the seventh European Community Framework Programme, an OPUS-21 no. UMO-

2021/41/B/ST8/03440 from National Science Centre, Poland (Radosław Barczak), an Australian Government

Research Training Program (RTP) scholarship (Ruth Fisher) and the CRC for Low Carbon living project RP2008

(support by the Cooperative Research Centres program, an Australian Government initiative).

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Importance of Musty Odour Character in the Emissions from Anaerobically Stabilized Dewatered Biosolids. Towards the Identification of Unknown Odour Components.