Report No DWI0486
Review of Operational & Experimental Techniques for the Removal of Bacteria, Viruses & Pathogens from Sewage Effluents
DWI0486
Sept 1988
EXECUTIVE SUMMARY
Pressure for compliance with the EEC Bathing Water
Quality Directive has prompted investigation into methods of
disinfection for marine discharge of wastewater. Disinfection may be an
alternative to discharge through long sea outfalls or may provide a
temporary solution while outfalls are constructed. Chlorination is the
established disinfectant for both water (eg. UK) and wastewater (eg.
US), but increasing concern over its environmental impact has led to
development of alternatives including UV; ozone; lime; peracetic acid;
chlorine dioxide; bromine chloride and gamma irradiation.
The Department of the Environment commissioned CES
to undertake a comparative evaluation of disinfection techniques. The
objectives of the study were to review both current operational
techniques and experimental techniques for the inactivation of
bacteria, viruses and pathogens in sewage effluents, with particular
reference to marine discharges. The inactivation efficiency of the
various techniques was to be compared and the actual or potential
capital and operating costs evaluated. In addition, promising
techniques for future application and aspects of disinfection requiring
further research were to be identified.
The report covers firstly the properties of
individual operational or experimental disinfectants, reviewing their
production and application; chemistry; mechanism of disinfection;
inactivation efficiencies; factors affecting efficiency; and
environmental impact. Operational and cost data from pilot and full
scale trials, both reported in the literature and provided by UK Water
Authorities are presented. Subsequent sections of the report comprise a
cross comparison of disinfectant alternatives in terms of their
inactivation efficiency, environmental impact and cost; and a brief
summary of Water Authority attitudes to disinfection. Factors affecting
the choice of disinfectant have been discussed and areas for future
research identified.
- Operational disinfection systems
- Chlorine is the most widely used wastewater disinfectant; in
the US approximately 62% of total municipal wastewater is chlorinated.
Chlorine is applied either as elemental chlorine (a dense corrosive
gas), or as a hypochlorite compound (typically sodium hypochlorite
solution) supplied as a 14-15% solution or generated on-site from brine
or seawater using electrolysis (OSEC). Hazards associated with the
handling of chlorine gas makes the use of sodium hypochlorite solution
or OSEC preferable in densely-populated areas.
Chlorine reacts rapidly with ammonia and organic compounds to form
chloramines and chlorinated organics. High concentrations of ammonia in
wastewater give combined chlorine residuals. Despite a much slower
bactericidal activity than free chlorine, given sufficient contact
time, chloramines are equally effective disinfectants. However, both
free and combined chlorine show low inactivation efficiency for
resistant microorganisms such as bacterial spores and cysts.
Chlorine residuals (both free and combined) are acutely toxic to
aquatic organisms at low concentrations and are persistent due to their
stability. The US EPA has proposed stringent discharge requirements for
total residual chlorine (mean 7.4µg/l for saltwater);
dechlorination is necessary at many chlorination facilities. Certain
chlorinated by-products, eg. trihalomethanes (THMs) are carcinogenic
and there are significant adverse environmental effects associated with
chlorinated organics formed during wastewater disinfection.
Despite its disadvantages, chlorine remains one of the most
cost-effective disinfectants available with low capital and operating
costs of under 2p/m3.
It is also suitable for application to raw sewage, unlike ozone and UV.
Its ease of application makes it suitable for emergency disinfection or
as a temporary measure.
- Chlorination/dechlorination was used at over 5000 wastewater
disinfection plants in 1987. The two most common dechlorination
processes use sulphur dioxide or granular activated carbon (GAC),
though sodium bisulphite, sodium sulphite, sodium thiosulphate, sodium
metabisulphite and biotin may also be used. Sulphur dioxide gas
requires similar dosing equipment as chlorine and reacts with chlorine
residuals on an equal molar basis (approximately 1mg/l is required to
remove 1mg/l chlorine). A disadvantage is that excessive dosage of
sulphur dioxide leads to depression in pH and dissolved oxygen,
necessitating reaeration. The total cost of chlorination can be
increased by up to 30-50% with the addition of a dechlorination step.
Dechlorination has been shown to reduce both chlorine residuals and the
mutagenic activity of water. GAC dechlorination can reduce mutagenicity
in drinking water both by application before (removal of precursors)
and after (removal of organochlorine compounds) chlorination.
- Ozone is being used increasingly for wastewater disinfection in
the US and to a lesser extent in Europe. In 1987 there were 19
operational plants in the US and 2 small scale wastewater plants in
France. Ozone is an unstable gas which is generated on site by
electrical discharge through air or oxygen. It decomposes rapidly in
aqueous solution and under alkaline conditions hydrolyses to form the
OH radical, which is a powerful oxidant. Conditions for oxidation are
improved in acidic pH, through slower direct oxidation. Ozone is both
an efficient bactericide and virucide, characterised by its rapidity
and the low concentrations required. Typical ozone doses in practice
for secondary effluents are 10 mg/l for a contact time of 10 min.
Ozonation is currently only practicable on effluents treated to at
least secondary quality. Ozonation of lower quality effluents is
limited by the high ozone demand associated with high COD and SS
levels. One advantage over chlorine is that ammonia levels in
wastewater have little effect on disinfection efficiency. Though ozone
appears not to produce THMs and may even destroy a number of THM
precursors, it oxidises a wide range of natural organics in wastewater
and can lead to significant changes in the nature and concentrations of
certain organic compounds. Ozone destroys most of the non-volatile
organic constituents in wastewater but produces others; concentrations
of mutagenic micropollutants can be increased by ozonation.
Though highly toxic, the hazards from ozone are minimised because it is
utilised immediately. However, operators need protection by adequate
leak detectors as concentrations of ozone generated are in excess of
occupational exposure limits. Accident risks at ozonation plants are
low; in over 70 years of large scale disinfection of potable water in
Canada and Mexico no fatalities have been reported. Costs are largely
generating equipment costs and the energy costs involved in ozone
production; approximately 30% of total costs arise from amortisation of
capital and 17% from power consumption. Comparison of operational
wastewater disinfection plants in 1984 found that capital and operating
costs for ozonation were approximately 2.5 and 2 times those of
chlorination/dechlorination respectively.
- Ultra-violet (UV) disinfection systems were operating at 53
wastewater plants in the US and Canada in 1984, with 64 in the design
or construction phase. These facilities operate sucessfully on
secondary effluents with flow capacities from 2.7m3/d - 2.2m3/s.
Wastewater quality is a major limiting factor because high
concentrations of solids can absorb UV and protect microorganisms by
encapsulation. Secondary treatment is necessary for UV to be both
technically and economically feasible. Factors governing the efficiency
of disinfection are UV dose (a product of lamp intensity and contact
time) and wastewater quality. Typical operational dosages for secondary
effluents range from 30 to 50 mWs/cm2. Inactivation of
viruses requires 3-4 times the dose for bacteria, whilst bacterial
spores and cysts are up to 9 and 15 times more resistant. Comparison of
UV dose with chemical dosages is difficult but the range of UV doses
for different pathogens appears narrower than that of chlorine. It is a
more efficient virucide than chlorine.
An advantage of UV irradiation is that it produces no harmful
by-products and causes only slight chemical changes in non-volatile
organics in wastewaters. UV does not produce a lasting residual thus
there is no residual toxicity to marine organisms. There is no economy
of scale for UV because the effluent flow rate is proportional to the
number of lamps. The major capital outlay is for UV lamps, while
operational costs arise from lamp replacement and electricity
consumption. Capital costs tend to be higher than those of a
dechlorination plant whilst operating costs are of the same order of
magnitude.
- The Clariflow single-stage lime treatment process was developed
in the UK by Blue Circle Industries plc in conjunction with Southern
Water and Portsmouth Polytechnic. The first plant has been in operation
at Sandown, Isle of Wight, since 1985 and successfully treats up to
21,000m3/d of raw, screened sewage. A patented lime-based
slurry facilitates the production of an upflow sludge blanket. A
minimum pH of 10.5-11.0 is required to achieve adequate coliform
disinfection, although at lower pH improved removals of BOD, SS and
metals are observed. A high pH regime has been shown elsewhere to be
effective for inactivation of other pathogenic microorganisms including
viruses.
The effluent is necessarily of a high pH, which may have a localised
ecological impact at the site of discharge. The process also generates
a large volume of sludge which must be dewatered and disposed of either
to agricultural land or, as at Sandown, to landfill. Capital costs are
high, although these may decrease for subsequent plants. Operating
costs are high, typically 5 times those of chlorine. Costs rise
disproportionately with pH, therefore it has been suggested that a
lower pH regime may be operated in winter when bacterial quality is
less critical.
- Experimental disinfection systems
- Chlorine dioxide has had extensive use as a water disinfectant
in Europe and -the US but has yet to be used as a wastewater
disinfectant. It is both a powerful bactericide and virucide even at
high pH levels and has an important advantage over chlorine in that it
does not appear to produce THMs.
Chlorine dioxide is a yellow explosive gas produced in situ from the
reaction of sodium chlorite with either chlorine gas or hydrochloric
acid. Even when generated on site there are hazards involved in
chemical handling, although development of a new method of production
under vacuum eliminates the explosive hazard. Chlorine dioxide is a
more powerful oxidant than chlorine and is also a more effective
virucide. Use of indicator organisms may thus yield conservative
performance data for its disinfection of secondary effluent. Chlorine
dioxide dosages of 0.05-1.0 times and residuals of 0.3-0.1 times that
of chlorine achieve equivalent bacterial reductions. Though THMs are
not formed, chlorine dioxide can react with organics to yield other
potentially hazardous chlorinated or unchlorinated by-products, some of
which are known carcinogens. Potential environmental impacts on marine
biota are not well documented. Chlorine dioxide is technically feasible
as a wastewater disinfectant, as demonstrated by short term operation
at a full scale wastewater plant, but operational costs are high
because of the price of the feed chemical. Capital costs are comparable
to chlorine, with only a slight modification of generation and feed
equipment necessary. Total costs tend to be 2-5 times as high per m3 as chlorine.
- Peracetic acid (PAA) exists as an equilibrium mixture with
hydrogen peroxide, acetic acid and water and is produced in the UK,
solely by Interox, under the trade name Oxymaster (12% w/w PAA).
Application of PAA is simple and direct and it is suitable for
disinfection of all sewage types. Whilst it has been shown to be an
efficient bactericide at concentrations of 15-20 mg/l PAA and 2 min
contact time; it is less effective as a virucide and sporicide, with
dosages of 100 mg/l and contact times of 30 min required for viral
inactivation.
The constituents of Oxymaster are known to cause toxic and mutagenic
effects, but its rapid reaction and dissipation means that significant
residuals are unlikely to occur. The highly oxidative nature of PAA
makes it capable of reacting with organic compounds in wastewater;
there is concern over the possible formation of epoxides and (through
free chlorine formation by peroxide radicals) chlorinated organic
compounds. Concentrations might be expected to be low but there are
currently inadequate data to evaluate the significance of this aspect.
By-products were not detected in seawater during disinfection trials
with PAA. Although capital costs for PAA are low, the high feed
chemical costs means that operating costs may be up to 6 times those of
chlorine per m3 of sewage treated.
- Bromine chloride is an interhalogen compound formed from an
equilibrium mixture of bromine and chlorine. It is hazardous at low
concentrations; although the reliability of storage and dispensing
methods have improved, this has been a major constraint in its
widespread application. Hydrolysis products of bromine chloride react
with nitrogenous compounds to form bromamines (analogous to chloramine
formation by chlorine). The inorganic bromamines are more effective
bactericides than inorganic chloramines while organic bromamines also
display some disinfectant properties. Though chlorine and bromine
chloride are equally effective as bactericides, bromine chloride is a
more efficient virucide, particularly in the presence of organic or
inorganic interfering substances.
Bromine-chloride is less toxic to fish than chlorine because of the
shorter half-life of its residuals. However, formation of brominated
THMs may pose more serious problems than the chlorinated analogues.
Brominated organic compounds can accumulate in fish exposed to
effluents disinfected with bromine chloride. Bromine chloride requires
much the same handling and dosing equipment as chlorine but due to the
higher rate of reactivity of bromine chloride, smaller contact tanks
may be used thus reducing capital costs. Operating costs are reportedly
of the same magnitude as those for the chlorination/dechlorination
process.
- Ionising radiation has been investigated as both a sludge and
wastewater disinfectant; it is already used in over 30 countries to
increase the shelf life of food. Radiation energy can be produced from
an electron accelerator or a gamma-radiation source such as Co-60 or
Cs-137. Energised electrons produce a much higher dose rate than Co-60
or Cs-137 sources but, unlike gamma radiation sources, have a
relatively short penetration range. Although effective in the
inactivation of most pathogens, the safety aspects and high costs
involved are serious drawbacks. Costs arise from the handling and
shielding equipment and source replenishment. Gamma-radiation tends to
be cheaper than electron acceleration at small facilities due to the
investment costs of a high voltage accelerator. Although there is
considerable industrial experience in the US with the use of
gamma-radiation sources, radiation is inherently hazardous and requires
proper protection and shielding facilities.
- Possible disinfection techniques
Embryonic techniques considered included bromine; heat; sensitized
photo-oxidation; quaternary ammonium compounds; activated carbon
adsorption; protein precipitants; filtration; and ultrasound. Although
several have been evaluated at pilot scale for the disinfection of
wastewater, technical limitations (filtration, ultrafiltration) or cost
considerations (activated carbon, heat) mitigate against their use at
full scale. None were considered to provide a viable technique which
could be developed for application within the next 5 years, although
ultrasonication could be a useful pretreatment method for ozonation and
UV irradiation.
- Comparisons of alternative disinfectants
Comparative summaries of both operational and experimental disinfection systems are given in Tables 1 and 2.
- In considering inactivation efficiency, it appears that the
most effective disinfectants in terms of the range of doses required
for bacterial and viral inactivation are chlorine dioxide followed by
ozone (see Figure 1). UV and gamma-irradiation also appear effective,
although data on viral inactivation in wastewaters are limited. The
halogens, bromine chloride and bromine, are more effective than
chlorine as virucides, though as bactericides their efficacies are
comparable. PAA requires high doses to be effective as a virucide.
Chlorine, both free and combined, is an efficient bactericide but its
use as a virucide appears limited. The limited information available on
lime treatment suggests that the Clariflow process is likely to be
effective for virus inactivation.
- The potential for residual toxicity effects on the marine
environment is dependent on the half-life of the residual. Chlorine has
the greatest hazardous potential, although the use of dechlorination
with sulphur dioxide can attenuate toxicity. This is particularly
important in relation to discharges to small rivers (as in many cases
in the US), where the capacity for dilution is much less than for
marine discharges. Bromine residuals are generally shorter-lived and
both bromine and bromine chloride have been shown to be less toxic than
chlorine. Whilst PAA may have a toxicity equal to that of chlorine, it
is unlikely to have a significant effect in practice because it is
unstable and short-lived. Although demonstrably toxic, ozone appears to
present few problems regarding residuals, due to its short half-life
and rapid dissipation from water. There is little available information
on UV, which cannot exhibit a direct toxic effect but may cause a
physical alteration of certain compounds and hence an indirect toxic
effect.
- By-product formation by chlorination is unequivocal and health
hazards of eg. carcinogenic THMs, are widely documented. The
environmental effects of wastewater chlorination have been demonstrated
in trials by Welsh Water, where bioaccumulation of chl orinated
organics in marine biota was observed. However, a decline following
termination of disinfection may have implications for seasonal
disinfection, suggesting that there may be potential for a degree of
purification of the biota during non-disinfection periods.
The effects of by-products formed by alternative disinfectants are
still largely unquantified. Whilst the strongly oxidative nature of
ozone, PAA and chlorine dioxide may be expected to alter the components
of the effluent matrix, many of the oxidation products do not appear to
be harmful. By-products of concern are aldehydes, hydroperoxides and
mutagenic activity of ozone; peroxides (possibly mutagenic) and small
amounts of chlorinated organic compounds by PAA through generation of
free chlorine; and brominated organics by bromine and bromine chloride.
Generation of halogenated organics may occur with chlorine dioxide if
free chlorine is present, although chlorine dioxide itself does not
form THMs. There is no evidence for mutagenic activity of chlorine
dioxide. Gamma-irradiation would be anticipated to cause mutagenic
activity, but there is insufficient operational experience to provide
evidence for this in practice.
Due to the lack of long-term operation of any of the disinfection
processes other than chlorine, evaluation of their hazardous effects
remains largely unresolved. However, whilst the absence of any
deleterious effects seems unlikely, it would seem that the
environmental impact of disinfectants such as ozone and UV will be of a
lesser magnitude than that arising from the use of chlorine compounds.
- The relative costs of the different processes as reported in
the literature are specific to a particular plant size. For example,
chlorine shows distinct economies of scale and whilst there is no
technical limit on the size of a UV plant, the costs tend to become
prohibitive at larger facilities because of the high operating costs.
On an economic basis, chlorine, hypochlorite and UV (at small-medium
plants) are the most cost effective. However this does not take into
account any remedial costs for precluding adverse environmental
effects, eg. by dechlorination. A dechlorination system, necessary to
prevent residual chlorine toxicity, adds 30-50% to the costs of
chlorine and sodium hypochlorite systems, and makes them comparable
with the costs of the halogens such as bromine chloride and chlorine
dioxide. Also additional benefits of a process such as colour removal
with ozone or phenol destruction with chlorine dioxide, are not
accounted for in a simple cost analysis. Ozone is generally reported to
be some 2-8 times more costly than chlorine, sodium hypochlorite
approximately equal to chlorine, and chlorine dioxide and
bromine/chlorine up to two times as expensive as chlorine.
Costs obtained from UK manufacturers for a theoretical case study based on a population of 25,000 and a DWF of 4,500 m3/d
are shown in Table 3. From this it can be seen that the costs of
disinfecting crude sewage, for which not all systems are appropriate,
increase according to the following series;
Cl2(gas) < Cl2(hypo) < Cl2(OSEC), PAA < Clariflow.
As with all chemical disinfectant systems, there is greatest
sensitivity of cost to dose. The costs given for chlorine are based on
a low dosage of 20 mg/l typically used in pilot studies but estimates
are also given for a higher dosage of 100 mg/l, as used at a full-scale
UK plant. Chlorine gas and hypochlorite remain the cheapest options
even at higher dose rates; whilst the cost of OSEC approaches that of
PAA. OSEC is significantly cheaper than the Clariflow process, which is
the most expensive option with a total cost approaching that of
land-based treatment. However, the Clariflow system does have
additional benefits such as removal of BOD, 55 and heavy metals. There
are insuffient data to evaluate the costs of ozonation of crude sewage
because it is generally only used for treated effluents. Assumed costs
based on the latter are intermediate between PAA and Clariflow.
For the disinfection of secondary effluents, costs increase as follows;
Cl2(gas) < Cl2(hypo) < Cl2(OSEC) < UV, PAA < 03
There is a smaller range between alternatives for treated effluents; UV
appears attractive particularly if compared with chlorination plus
dechlorination, and is of a similar cost to PAA. Ozone remains one of
the most expensive options. The comparative magnitude of process costs
obtained in the case study is similar to that reported for operational
plants absolute costs vary with plant size and effluent quality.
- Water Authority attitudes to disinfection
A summary of disinfection trials carried out by Water Authorities is
given in Table 4. The general concensus among Water Authorities would
appear to be to opt for long sea outfalls wherever possible. Even with
adequate screening and disinfection, the public acceptability of
swimming in sewage polluted waters is questionable. Although there is a
great deal of interest in the potential for disinfection, there is a
general reluctance for its adoption on a long-term basis because of the
lack of proven efficacy and the high operating costs. The awareness of
environmental impact observed by water authorities, both in-house and
increasingly amongst consumers, also tends to mitigate against
disinfection. It is however being seriously considered, as for example
by South West Water, as a short term measure while outfalls are
constructed.
- Conclusions
There are several processes available which can disinfect sewage to
achieve coliform levels permitting discharge through comparatively
short outfalls. Estimates of UK costs show prices ranging from
approximately 1.6 p/m3 to 19 p/m3 for year-round disinfection or 2.0 p/m3 to 27 p/m3
for six monthly operation. These costs would reduce slightly for very
large outfall schemes, and in some cases such as Clariflow, are
associated with additional benefits such as increased BOD, 55 and heavy
metal removal. In this respect (although not included in the present
remit), it would be of interest to compare the costs of disinfection
options with those for a long sea outfall. The figures presented should
also be viewed in the context of the costs levied by Water Authorities
for full biological treatment for subsequent discharge to an inland
watercourse, typically approaching 25 p/m3. In relation to
this, those processes at the lower end of the cost range indicated may
be attractive if they can be shown to be environmentally acceptable.
A major drawback with disinfectants is the formation of hazardous
by-products. It is difficult to interpret existing information in the
context of the marine environment rather than in relation to human
consumption, since much of the toxicological data for chlorinated
by-products relates to exposure via drinking water. The processes with
the least by-products are either very costly or not presently effective
for raw sewage. The only option without associated by-products is a
long sea outfall. Until further evidence proves otherwise, it is
considered that disinfection by chemical means should only be used on a
short term basis, while-an outfall is being constructed. It is
interesting to note the apparent recovery of marine organisms from the
effects of chlorinated discharge in the Welsh Water study; this
suggests that seasonal disinfection may be less environmentally
detrimental than year-round treatment. Nevertheless, the possible
establishment of reservoirs of pathogens in sediments during the winter
months would have to be reviewed at individual locations.
Where standards are imposed rigorously and beaches have to remain open
for five years or so until an outfall scheme (or land-based treatment)
comes on-stream, a choice of disinfectant may well have to be made. If
economic considerations prevail, chlorine compounds (specifically
hypochlorite) are attractive. Seasonal application and dechlorination
to preclude residual toxicity could be used to reduce environmental
impact. However, experience at Weston-Super-Mare, where dosages at the
upper end of the chlorine range in Table 3 are applied and compliance
is still not always achieved, has to be borne in mind. Although about
three times more expensive than gaseous chlorine, the lower degree of
by-product formation associated with chlorine dioxide suggests that it
merits further investigation as a wastewater disinfectant.
If economic aspects are not overriding and minimal environmmental
impact is considered important, the Clariflow process could have
application. However, construction time, land acquisition, sludge
disposal and an overall cost approaching that of land-based treatment
would have to be considered. Ozone is a potentially attractive option
in economic comparison to Clariflow, but its efficacy on crude sewage
is not well documented. The intermediate option in terms of both cost
and likely impact is PAA, however it is not possible to assess the
latter factor adequately on the basis of current data.
An examination of embryonic technologies for sewage disinfection
identified no systems which showed sufficient development potential for
use within the next five years. Aspects identified for further research
include the following:
- a detailed review of existing installations and operational
experience in the US, particularly with regard to environmental effects;
- more comprehensive studies on the efficacy of alternative disinfectants for viral inactivation;
- an evaluation of effects of the use of viral indicators and of
the phenomenon of viable, non-culturable cells on the apparent
efficiency of disinfectants;
- field trials to assess the degree and significance of
by-product formation by disinfectants, particularly chlorine, chlorine
dioxide and PAA;
- a practical investigation of the applicability of chlorine dioxide for UK marine discharges.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.