Table Of ContentAdvances in Water Pollution Control
A Series of Conferences Sponsored by the International
Association on Water Pollution Research and Control
Editorial Office lAWPRC, 1 Queen Anne's Gate,
London SWIH 9BT, UK
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E. J. Izod Publications Manager
P. T. Nagle Technical Sub-editor
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Scíentífíc Programme Committee
P. Cooper {Chairman) UK A. Wood South Africa
K. Bucksteeg FRG T. Davies Australia
H. Brix Denmark T. Parr UK
R. Haberl Austria A. Lienard France
D. Hammer USA N. Lowe UK
Organising Committee
P. Cooper {Chairman) UK J. Hobson UK
K. Bucksteeg FRG E. Porter UK
A. Milburn UK B. Green UK
J. Busing Belgium C. Coombes UK
R. Clayton UK J. Christian UK
Sponsoring Organisations
Water Research Centre
European Communities Directorate General for Science, Research
and Development
European Water Pollution Control Association
International Association on Water Pollution Research and Control
CONSTRUCTED WETLANDS IN
WATER POLLUTION CONTROL
Proceedings of the International Conference on the Use of Constructed
Wetlands in Water Pollution Control, held in Cambridge, UK,
24-28 September 1990
Editors
P. F. COOPER and B. C. FINDLATER
WRc Swindon, Wiltshire, UK
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Copyright © 1990 International Association on Water Pollution
Research and Control
All Rights Reserved. No part of this publication may be reproduced,
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First edition 1990
Library of Congress Catalog Card No. 82-645900
ISBN 0 08 040784 6
In order to make this volume available as economically and as rapidly
as possible the author's typescript has been reproduced in its original
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it is hoped that they in no way distract the reader.
Printed in Great Britain by BPCC Wheatons Ltd, Exeter
FOREWORD
In the past 5 years the use of hydrophyte-based systems for sewage, mining and industrial wastewater
treatment has received much attention because of their potential for providing cost-effective treatment for
small communities. The first interest in these systems in the United Kingdom occurred in 1985 and this led
to the formation of the national Reed Bed Systems Co-ordinating Group. It was soon realised that there was
already considerable experience (good and bad) with these systems in other European countries, particularly
Germany and Denmark. Informal arrangements to exchange information led in October 1986 to the
decision to form a European co-ordinating group. This group comprises representatives from 10 countries
and is recognized by the Commission of the European Communities (CEC) and the European Water
Pollution Control Association (EWPCA) as an expert contact group. When the group was set up, two of the
objectives were to produce Design and Operations Guidelines and to organise a Conference in 1990 to draw
together all the leading experts in the field. A very successful conference was organised by Tennessee
Valley Authority at Chattanooga, Tennessee in June 1988. That conference attracted 600 delegates and it
was felt appropriate to organise another one to document the progress that has been made in this fast-
growing field in the last 2 years.
The response to the Conference has been very good indeed. These preprints contain nearly 50 papers that
will be presented orally by authors from 12 countries. In addition there are more than 20 poster papers
included. It is expected that about another twenty poster papers, not included in these preprints, will be
presented at the Conference.
The Conference is being organized by the Water Research Centre and the UK Reed Bed Treatment Systems
Co-ordinating Group on behalf of the EG/EWPCA Emergent Hydrophyte Treatment Systems (EHTS)
Expert Contact Group. It is being co-organised by the Commission of the European Communities
Directorate-General for Science, Research and Development, the European Water Pollution Control
Association (EWPCA) and the International Association on Water Pollution Research and Control
(lAWPRC).
I take this opportunity to thank all the many people who have contributed to the success of this conference.
I would like to thank the authors, referees and session chairmen. In particular I would like to thank the
members of the UK Reed Bed Treatment Systems Co-ordinating Group, the EC/EWPCA EHTS expert
contact group and the lAWPRC Specialist Group on the use of Macrophytes in Water Pollution Control, for
all their help in developing the technical programme. Many thanks also go to my WRc colleagues, Eric
Porter the Conference Organiser and John Hobson and Carol Findlater for their help with a great deal of
editorial and refereeing work. My thanks go to Elizabeth Izod and Paul Nagle of the lAWPRC Editorial
Staff for all their help and forebearance in dealing with late manuscripts. I am grateful to Anglian Water,
Southern Water and Severn-Trent Water for allowing delegates to visit Reed Bed Treatment Systems on
their sites at Acle and Freethorpe, Gravesend and Little Stretton respecdvely.
Finally, may I wish all authors and other participants a very good conference which I hope will contribute
greatly to the state of knowledge in this exciting field.
PAUL COOPER
Chairman of the Conference Organising Committee
Water Research Centre, Swindon, UK
July 1990
THE NEED FOR HYDROPHYTE-
BASED SYSTEMS IN THE
TREATMENT OF WASTE WATER
FROM SMALL COMMUNITIES
N. R. Lowe
Dwr Cymru Cyf., Cambrian Way, Brecon Powys, LD3 7HP, Wales, UK
ABSTRACT
Small communities are frequently remote and may be set in attractive
surroundings. Their wastewater treatment facilities must be capable of
operating effectively but road communications are often poor in rural areas
and there may be no proper access to the treatment works. An adequate power
supply may be lacking, and both regular maintenance by skilled personnel and
the removal and disposal of sludge may be difficult to achieve and expensive.
The need therefore exists for a treatment system which is economical and
reliable, preferably aesthetically attractive and which requires a minimum of
skilled maintenance and no power supply. It must, however, be capable of
sufficiently intensive treatment to meet relevant effluent quality standards.
The success of hydrophyte-based systems in treating wastewater from small
communities will be measured against such criteria.
KEYWORDS
Sewage treatment; small communities; effectiveness; low technology; minimum
maintenance; hydrophyte-based systems.
INTRODUCTION
Throughout the world a substantial proportion of the population lives in
small villages. Even in the relatively densely populated England and Wales,
for example, the Prospectus for the sale of shares in the water services
companies (1989) records that more than 2000 sewage treatment works each
serve a population of fewer than 250. Many of these communities are situated
more or less remote from large centres of population. The absence of major
roads and railways may make transport and communications difficult and there
may be a relative lack of services such as power supplies. It is also
frequently the case, at least in much of Western Europe, that areas with
sparse population are attractive in appearance. Such attractiveness is
increasingly recognised and defended by the local inhabitants, by
environmental and conservation organisations and by governments wishing to
foster tourism.
At the same time the view is gaining ground that all sewage effluents should
receive at least secondary biological treatment before discharge to inland
watercourses. In general the sewage from the larger settlements in Europe is
already collected and effectively treated but appropriate provision still
Ν. R. LOWE
needs to be made for many smaller villages as well as other communities such
as schools, hotels and caravan sites which may not be connected to public
sewers. There is therefore a substantial need for sewage treatment
facilities for these communities.
PROCESS REQUIREMENTS
Whilst many requirements for sewage treatment are constant whatever the size
of the community, in some respects the treatment of sewage from small
populations presents particular problems. It is therefore useful to look at
the requirements for a small sewage works before considering the criteria
used to judge a successful installation.
Effectiveness
The degree of treatment needed will vary substantially within and between
countries. However it is convenient to consider four broad levels of
treatment.
The most basic secondary treatment level involves the removal of labile
dissolved BOD together with a proportion of the primary and secondary solids.
This minimum level of secondary sewage treatment is likely to be appropriate
when sewage effluent is discharged with high dilution into a watercourse with
no pollution problems. The next level of treatment enables the removal of
higher levels of BOD and solids to comply with stringent effluent standards
but where no requirement exists for the removal of ammonia or nutrients. For
discharges to sensitive watercourses, particularly with little available
dilution it may additionally be necessary to achieve nitrification, that is
the oxidation of ammoniacal nitrogen. Finally in areas where eutrophication
is prevalent it may be that the removal of significant quantities of nitrogen
and/or phosphorus is also required.
Sewage Characteristics
The characteristics of the sewage to be treated vary widely from country to
country. For example, surface water may be collected together with or
separately from the foul flow. Sewage strength will also be affected by the
per capita consumption of water. Variation of flow from small communities
within a 24 hour period is likely to be high particularly where, as is
frequently the case, sewer lengths are short and no sewage pumping is
involved. In such cases a significant proportion of the total flow is likely
to occur during relatively few hours in the day and night flows may be close
to zero. Another factor which can cause periodic flow variation,
particularly in scenically attractive areas, is tourism. The contributory
population, and therefore flow, can be many times greater in the peak holiday
season than when few or no tourists are present.
The strength of the incoming sewage will also vary for demographic and
climatic reasons. However, in general it is likely to be domestic rather
than industrial in nature, and therefore to be both biodegradable and
relatively free from elevated levels of toxic constituents.
Operations and Maintenance
The factors which influence the method of operation of a sewage treatment
process from a small population in a remote and rural area may differ
markedly from those in an industrial town or city. However a common factor
is that the process must function reliably in order to comply with legal
standards. These are likely to be either emission standards governing the
quality of effluent or descriptive standards relating to the type of process
Hydrophyte-based systems
employed and the effectiveness of its operation and maintenance.
It therefore follows that the chosen installation must be capable of
performing effectively and reliably within the context of the maintenance
regime available locally. In rural areas such maintenance is usually carried
out in one of two ways, depending to some extent on the country in question.
In the UK it is usual to employ mobile teams of operators who work full-time
in maintaining a group of sewage works which may cover a wide area. In order
to lower costs there is in such circumstances clearly an incentive to reduce
the frequency of works visits and a local study has been undertaken to
establish the effect of such a reduction (Wall et al., 1984). This study
concluded that sludge removal and very basic operational practices such as
clearing filter distribution orifices were the overriding priorities in
maintaining effluent quality and should dictate the frequency of visits.
Other maintenance was only necessary at less frequent intervals. It
therefore follows that if the critical operations can be eliminated, or the
frequency with which they need to be carried out can be reduced, lower
operating costs can be expected.
In France it is common practice for local, unskilled operators to assume
responsibility for the village works. Since they frequently live close to
the works it is relatively easy for them to visit the works daily for a short
period to perform simple tasks. Under these circumstances there is less
incentive to choose a process requiring infrequent visits. However, in
either case there will be a relative scarcity of resources available for the
skilled maintenance of complex mechanical equipment.
Site Constraints
Of immediate importance in any project appraisal is the first cost of
building and installation. However for a small works, this is likely to be
relatively small in absolute terms and may not be the all-important and
limiting factor that it could be in selecting a large works process.
In a rural area the availability of land is likely to be comparatively high
and its cost correspondingly low. As a consequence the quantity of land
utilised is frequently less of a consideration than it may be elsewhere.
This land is, however, more likely to be subject to aesthetic constraints in
the rural than the industrial landscape. Restrictions may be placed on the
appearance of the installation to ensure that it is in keeping with the
surrounding environment. In the case of a conventional treatment plant this
may necessitate building it underground, covering it or otherwise disguising
its appearance, usually at additional cost. There may also be increased
pressure to reduce the nuisance caused by smell, flies and noise if the
treatment works is to be sited in a particularly attractive and unspoilt
area. However in those remote areas which are not generally regarded as of a
high amenity value, these considerations may in fact be of less importance
than in a highly populated area.
The perception of public safety varies from one country to another but must
be taken into account when considering scheme options. In the UK, for
example, an open body of water such as a tank or lagoon is considered to be
particularly hazardous. This usually necessitates surrounding the water body
by high security fencing particularly when the water contains, or is derived
from, sewage.
Of great importance is the complexity of the installation and the level of
skill required in its manufacture and construction. In remote areas there
may be a relative shortage of capacity in process plant manufacture and civil
construction. Also, it may be difficult and expensive to provide a supply of
electricity to the site. Even where this can be provided it may well be
relatively unreliable particularly if a substantial length of vulnerable
above-ground cable is involved. The quantity and type of electricity
Ν. R. LOWE
available may also be inadequate for some processes. Ideally in such cases
the treatment processes would not require a power supply at all. These
considerations would all favour a "low technology" approach to sewage
treatment.
AVAILABLE METHODS
There are a limited number of processes in widespread use in Europe which
provide for the secondary treatment of domestic sewage. For many years the
two most commonly used processes have been activated sludge and percolating
filters. However the use of rotating biological contactors is now
widespread.
Effectiveness
All of the above methods can be operated to remove labile BOD, together with
suspended solids. In addition they can all, at somewhat greater cost, be
designed and operated to remove a high proportion of BOD and suspended solids
to produce effluents low in these constituents.
The ease with which substantially complete nitrification can be reliably
achieved depends for a given process on temperature. Percolating filters,
activated sludge units and rotating biological contactors can all be induced
at a cost to nitrify reliably even during North European winters. Also,
careful design and operation of activated sludge systems can achieve nitrogen
removal down to levels of 4-6mg/l total nitrogen. However such a complex
system would not normally be considered desirable for a small community.
With regard to phosphorus, it has been shown that consistent removal of up to
95% of the sewage content is practicable by means of chemical precipitation
(Harremoes, 1988; Kerrison et al., 1989). Again, however, sophisticated
control is necessary and this is not compatible with the low technology
approach that seems most suitable for small, isolated communities.
Percolating filters may suffer from one drawback in that they can dry out if
there is no flow for a prolonged period overnight. In the early morning
there is frequently a sudden increase in both flow and load such that daily
maximum values may be reached only a short time after the zero flow state.
Under such conditions, effluent quality may deteriorate, perhaps to the
extent of breaching emission standards.
More extreme conditions of flow and load variation occur when the treatment
facilities are provided for a temporary population. Examples include holiday
camps, caravan sites and military establishments. In these cases there may
be no sewage load at all for part of the year with the result that most of
the biomass required for secondary treatment dies out. Activated sludge
units can be re-commissioned by importing biomass from a nearby works but
percolating filters and rotating biological contactors can require a period
of weeks to recover full treatment effectiveness. If the full sewage load is
applied immediately, for example at the beginning of the holiday season,
effluent quality may deterioriate very substantially for an extended period.
Site Constraints
Activated sludge units are often regarded as having a relatively low initial
cost and a low requirement for land. However they require a substantial
power supply and can be a source of noise and aerosol sprays. Under such
circumstances they may be considered to be unsuitable for installation near
to houses or in environmentally sensitive areas. Percolating filters are
normally almost silent and if adequate static head is available, they may not
require a power supply. However they can be expensive in terms of first cost
and require a relatively large area of land. At certain times of the year
they can give rise to nuisance from flies whose larval stages develop on the
Hydrophyte-based systems
surface film of the filter media. Rotating biological contactors can be
relatively inexpensive to provide, require very little land and can be
installed underground to blend with the surrounding landscape. They are
generally quiet and do not give rise to a fly nuisance. They are therefore
usually considered to be acceptable close to human habitation and in
scenically attractive areas. However, again they require a suitable power
supply although this can be of more modest capacity than that required for
activated sludge.
Operations and Maintenance
In comparison with other treatment processes activated sludge units in
general require a degree of skill in their operation. Failure to control
mixed liquor suspended solids, dissolved oxygen and sludge return and wastage
levels can result in the process working at substantially less than its
maximum capacity. If the sludge does not settle in the final clarifiers,
very high levels of suspended solids may be discharged to the watercourse.
Consequently, the activated sludge process is often considered to be
unsuitable for installation in remote areas with limited operation and
maintenance resources, particularly when a consistently high quality effluent
is required.
Percolating filters by contrast do not require a high level of skilled
maintenance. Visits by operators, who may be unskilled, are necessary to
deal with blockages in small-bore pipes and in the filter distributors.
Dosing syphons, where fitted, may be a source of trouble. Also it has been
found that some package plants based on the percolating filter principle are
not sufficiently effective, reliable and easy to maintain to ensure their
suitability for installation in remote or sensitive situations (Stoodley,
1989) .
Rotating biological contactors fall somewhere in between in that a degree of
skilled maintenance is required for the mechanical and electrical equipment.
However there are few additional tasks for the unskilled workforce apart from
occasional desludging.
Sludge Production
The treatment and effective disposal of sludge in an environmentally friendly
manner is of great importance, since the associated cost is broadly
equivalent to that of the sewage treatment itself. In the UK, for example,
it has been estimated by the Institution of Water and Environmental
Management (1989) that around 1.2 million tonnes dry solids of sewage sludge
are produced each year and that the cost of its treatment and disposal
approaches £250 million. Also, as environmental constraints on disposal
routes increase, the treatment and disposal of sludge is becoming more
difficult and more expensive. Therefore the quantity of sludge produced and
its characteristics are a major factor in choosing a sewage treatment
process.
Percolating filters, activated sludge and rotating biological contactors in
general produce comparable quantities of sludge in treating domestic sewage.
In the UK and including any primary sludge this usually amounts to between
0.02 kg and 0.08 kg of dry solids of sludge per head of population per day,
depending on the degree of treatment provided. As a rule, installations
where the sludge receives digestion and stabilisation within the wastewater
treatment process will produce less sludge than others.
Removal of surplus activated or humus sludge from a small sewage treatment
works is normally required at a frequency of around once a week. Since this
may be the factor which dictates the frequency of works visit, any process
enabling sludge removal at longer intervals may allow this frequency to be
Ν. R. LOWE
reduced. Especially where groups of works are maintained by mobile teams
this is likely to result in savings in operating costs.
What Are We Looking For?
An ideal process for treating sewage from small communities in remote, rural
areas must satisfy criteria some of which will be different from those
relevant to sewage treatment in towns. All of the following features are
desirable, and some are essential:
effective and reliable sewage treatment to the desired standard
throughout the year
no power requirement
little maintenance and this to be undertaken as far as possible by
unskilled operators
the production of only a small quantity of sludge
infrequent desludging
attractive appearance to blend with the surroundings
no environmental nuisance such as noise, smell or flies
the ability to deal with varying diurnal flows and in particular to
resist drying out during periods of zero flow at night
the ability to deal with varying flows throughout the year and in
particular to achieve rapidly its full treatment capability after an
extended period without incoming sewage load.
Since conventional treatment systems are not entirely satisfactory in all
these respects, a great deal of work has been undertaken to develop
alternatives. Most of these are based on natural ecosystems and can be
regarded as employing "low technology".
LOW TECHNOLOGY ALTERNATIVES
A number of types of sewage treatment systems have been developed which do
not make use of mechanical plant and equipment. Oxidation ponds for example
are well established, especially in warm climates. There are well over 1000
installations in France, particularly in the south. They are recognised as
being capable of removing the labile BOD component of sewage under conditions
of limited maintenance by unspecialised employees. Average effluent quality
during the summer months, which is normally the season of maximum impact on
the watercourses due to lower dilution and high temperatures, was recorded as
31 mg/1 BOD by Boutin and his co-workers (personal communication). Oxidation
ponds have not been adopted in the UK, however, partly because of the climate
which is generally cooler and wetter than Southern France, but also owing to
aesthetic considerations. There is also an adverse reaction by the public to
large bodies of open water containing sewage and this has led to an
unwillingness to experiment with such a form of treatment without a preceding
secondary biological stage.
A development of pond treatment involves the use of floating macrophytes such
as duckweed (Lemna minor) and water hyacinth (gichhornia crassipes) (Brix and
Schierup, 1989). Nutrients are absorbed by the growing plants which are
regularly harvested. This process adds to the operating cost, however, and
the disposal of the harvested plants may cause difficulties. Nevertheless,
in countries where labour is easily available at low cost and particularly
