1. 46 Water & Sewerage Journal
Human norovirus
in untreated and
treated sewage
Carlos JA Campos
NOROVIRUS CONTROL
Water Quality Scientist, Centre for Environment, Fisheries &
Aquaculture Science (CEFAS)
Human noroviruses (NoV)
have been responsible for
millions of cases of
gastrointestinal disease in
the UK. These viruses make
headlines every two to four
years when new strains
emerge as the virus mutates,
usually during the months of
October to March.
In 2002/03, NoV outbreaks
in hospitals cost the NHS
around £115 million1
.
These viruses are excreted in high
numbers in the faeces of infected
individuals and may directly or
indirectly contaminate surface waters.
For this reason, wastewater treatments
are an important means for reducing
NoV transmission in the environment
outside the host and to prevent new
cycles of human infection. It is therefore
important to characterise typical
concentrations of NoV in untreated
sewage and treated effluents to
develop NoV risk-management
measures.
Research approach
To characterise the relative NoV risk
from untreated and treated sewage
discharges, samples were taken from
the different stages of the treatment
process at a full-scale sewage treatment
works (STW) for NoV testing
(genogroups I and II) using a PCR
method during the period October
2012 to January 2015. The treatment
process included screening, primary
settlement, activated sludge and UV
disinfection prior to discharge into an
estuary. The activated sludge process
is the Modified Ludzack-Ettinger (MLE)
process, which is a two-stage
biological nitrification/de-nitrification
with internal recirculation.
The N removal process consists of an
aerobic zone, in which nitrification
occurs, and an anaerobic zone in which
de-nitrification occurs. The nitrified flow
is fed back to the low oxygen anoxic
zones, which is de-nitrified by the
influent flow from the primary
settlement tanks. Samples were also
taken from the storm tank associated
with the STW.
Norovirus prevalence/
seasonality
During the study period, NoV was
frequently detected in samples of
screened influent (93 per cent of
samples positive for GI and 100 per
cent positive for GII), indicating that
NoV were frequently excreted from the
local human population. However,
there was extreme variability in the
concentrations detected. For GI,
concentrations ranged 2.3log10 orders
of magnitude, while for GII they ranged
2.9log10 orders of magnitude.
The mean concentrations of GI and GII
in influent samples taken during the
high-risk period were 925 genome
copies/ml and 10,025 copies/ml.
In settled stormwater samples, mean
NoV GI and GII concentrations were
513 copies/ml and 3,174 copies/ml,
respectively.
Mean concentrations of total NoV (GI
and GII summed) in the influent were
higher in 2012/13 (13,914 copies/ml)
than in 2014/15 (6,541 copies/ml).
This variation between years indicates
the general concentrations of NoV
As part of CEFAS’ recent research into norovirus risk, samples were taken at
different stages of the sewage treatment process at a full-scale works as well as
the associated storm tank

2. Water & Sewerage Journal 47
arriving at STW that reflect the
proportion of human population
infected and are probably associated to
the emergence of a new strain (GII.4
Sydney2012) across England, causing
a peak number of cases relative to
previous seasons2
.
MLE effective in NoV
reduction
All the physical and biological sewage-
treatment processes are expected to
remove or inactivate NoV to some
extent. However, none of them were
observed to reduce NoV to a similar
extent. The concentrations of NoV GII
in influent (screened) were significantly
higher than those in primary settled
effluents and in the activated
sludge tank.
The MLE process was tmore effective
than primary settlement in removing
NoV (see Figure 1). Other studies have
found higher efficiency of activated
sludge in reducing NoV than other
treatment processes, such as trickling
filters. This is probably associated with
the short contact times between the
viruses and the filter media and/or
the elution of viruses in the trickling
filters relative to the longer contact
times characteristic of suspended
growth processes.
Factors influencing
norovirus removal
NoV GII removal increased as the
concentration of the virus in the influent
also increased (see Figure 2). Linear
modelling showed that an increase in
influent concentrations by a factor of 10
would correspond to an increase of
0.5log10 virus removal. This indicates
that there is large variation in STW
performance, with respect to NoV
removal within the range of normal
operating conditions of the STW, and
therefore large scope for enhancement
of NoV removal in these types of
treatment processes.
It is possible that the PCR methodology
used for NoV measurement
underestimated the inactivation of the
virus during UV disinfection, because
PCR detects virus genome material
which may potentially still be
detectable for a period following virus
inactivation. Therefore, further studies
comparing PCR and virus culture
methods applied to UV disinfection of
wastewater would help inform this
possibility. Based on the results
obtained, the highest risk to public
health is associated with storm
overflows, even during periods of low-
virus prevalence, because the
infectious dose of the virus is very low
and its resistance to environmental
stressors is high. The public-health
significance of storm discharges has
been demonstrated by associations
between rainfall events, sewage
bypasses and increased frequency of
hospital admissions for treatment of
gastrointestinal illness3
and reporting of
NoV outbreaks linked to consumption
of oysters harvested from areas
impacted by untreated sewage spills4
.
References
1. Lopman, BA; Reacher, MH; Vipond,
IB; Hill, D; Perry, C; Halladay, T; Brown,
DW; Edmunds, WJ; Saranji, J (2004).
Epidemiology and cost of nosocomial
gastroenteritis, Avon, England, 2002-
2003. Emerging Infectious Diseases
10(10): 1827–1834.
2. Allen, DJ; Adams, NL; Aladin, F;
Harris, JP; Brown, DWG (2014).
Emergence of the GII-4 norovirus
Sydney2012 strain in England, winter
2012–2013. PLoS ONE 9(2): e88978.
3. Redman, RL; Nenn, CA; Eastwood,
D; Gorelick, MH (2007). Pediatic
emergency department visits for
diarrheal illness increased after release
of undertreated sewage. Pediatrics
120(6): e1472–e1475.
4. Wall, R; Dymond, N; Bell, A;
Thornley, C; Buik, H; Cumming, D;
Petersen, N (2011). Two New Zealand
outbreaks of norovirus gastroenteritis
linked to commercially farmed oysters.
New Zealand Medical Journal
124(1347): 63–71.
Further reading
Campos, CJA; Avant, J; Lowther, J; Till,
D; Lees, D (2013). Levels of norovirus
and E coli in untreated, biologically
treated and UV-disinfected sewage
effluent discharged to a shellfish water.
Journal of Water Resources and
Protection 5: 978–982.
Campos, CJA; Lees, DN (2014).
Environmental transmission of human
noroviruses in shellfish waters. Applied
and Environmental Microbiology
80(12): 3552–3561.
Contact
E: carlos.campos@cefas.co.uk
Figure1:Removalofnorovirus
genogroupsI(GI)andII(GII)throughthe
sewagetreatmentprocess
Figure 2: Norovirus removal through the treatment process (log10 scale) as a
function of the concentrations in the influent