2. Environmental impacts and protective measures

Contents - Previous - Next

2.1 Introductory notes

In spite of the basically environment-oriented objective of wastewater disposal, various problem factors may arise which may be impossible or difficult to overcome:

a) technically/economically unavoidable emissions (residual emissions), from the wastewater disposal installations which have an overall impact on air, soil and water, on people and on ecosystems

b) unforeseen increase in volume of sewage from dwellings (due to changes in lifestyle)

c) unforeseen increase in volume of sewage from commercial and industrial establishments (due to production increases, fluctuations, seasonal operation)

d) eutrophication phenomena in the receiving body of water into which the treated wastewater is discharged, during long periods of low rainfall

e) adverse consequences of using sewage sludge or composted refuse/sewage sludge in agriculture for the purposes of recycling.

From the start, reasonable allowance should be made for the above-mentioned problem factors in all project management activities, in order to minimise from the outset any conceivable effects, using suitable measures of an organisational, structural and operational nature, and in certain circumstances also with recourse to emergency measures. Furthermore, wastewater disposal measures are to be planned taking into account local conditions so that they satisfy the generally accepted rules of wastewater technology or the state of the art (after careful consideration where necessary).

Considering each relevant sub-sector of wastewater disposal in turn, the potential typical environmental impacts are as follows (7), (8).

2.2 Typical environmental impact

In any wastewater disposal project, it is vital to decide whether to adopt:

- a decentralised local sewerage system (individual sewerage system at each source with wastewater collection pits, cesspits or small sewage works, latrines, etc.) or
- a central local sewerage system (collective sewerage system comprising a sewer network with all necessary installations, to collect, divert and deliver the wastewater produced at the individual sources to one or more (central) sewage treatment works(s)

Different environmental effects occur here, the most important of which are set out below.

2.2.1 Impact of wastewater collection and removal

2.2.1.1 Decentralised sewerage system

Decentralised wastewater disposal may have the following adverse effects on the environment:

For the user, individual sewerage systems mean higher expenditure on maintenance and upkeep compared with systems connected to a central sewerage system. If not properly constructed this can lead to problems such as

- poor quality of the effluent from the small sewage works (if the sludge settling chambers are not regularly and properly emptied) and hence the contamination of the receiving body of water,
- frequent running of emergency overflows from pump shafts if the pumps are not properly maintained and hence contamination of areas which should only receive effluent in definite cases of emergency,
- contamination of the subsoil and particularly of groundwater in the case of dry closets (aerated/unaerated latrines), cesspools, percolation systems (particularly soakaways after aerobic or anaerobic biological wastewater treatment) leaking cesspits etc. in particular where the hydrogeological conditions are unsuitable,
- health hazards due to the operation of individual sewerage systems (e.g. danger of infection through direct contact when emptying pit contents; through insects and rat infestation),
- health hazards upon final disposal (discharge) of sludge from small sewage works or pit contents from collection cesspits, where not properly executed,
- aesthetic and odour nuisance,
- no possibility of central removal and treatment of commercial/industrial wastewater together with domestic sewage.

Beneficial effects of decentralised wastewater disposal on the environment may be as follows:

- The natural water cycle is scarcely interrupted or disturbed by the proper collection and removal of rainwater (this considerably reduces the amount of percolation from rainwater).
- There may be a greater incentive to reduce water consumption (increased consumption will lead to substantially higher costs of removal of pit contents).
- (Inefficient) receiving bodies of water are not subject to sporadic or sudden contamination at the rainwater overflows and outfalls or at the outfall works of sewage treatment plants.
- Eutrophication or desertification phenomena in receiving bodies of water are largely eliminated.

2.2.1.2 Central sewerage system

The beneficial effects of a decentralised local sewerage system mentioned in 2.2.1.1. are absent in the case of a central local sewerage system (central wastewater disposal). Indeed, their absence is a positive disadvantage. In addition, a central sewerage system has the following adverse effects on the environment:

- With inadequately designed or manufactured pipe couplings, serious leakage may occur (penetration of groundwater; hydraulic overload of pipes, pumping station and sewage works; leakage of wastewater; contamination of subsoil or groundwater).
- With large wastewater pumping stations, noise and odour nuisance may occur if

they are too close to neighbouring buildings or

sound insulation, aeration, ventilation and deodorization are lacking or inadequate.

Major positive effects of a central sewerage system are in particular the following (9):

- protection of the population from health hazards caused by infectious germs transmitted directly or indirectly by water, e.g. in the event of contamination of groundwater used for individual drinking water supplies or by direct contact with the wastewater.
- protection of the population from aesthetic nuisance caused by substances in wastewater which readily putrefy.
- protection of the population from flooding of cellars and storage rooms during storms.
- safeguarding of motor vehicle, bicycle and pedestrian traffic, even in case of heavy rainfall.
- possibility of removal and treatment of commercial or industrial wastewater together with domestic sewage.
- protection of utilisable groundwater reserves from contamination by substances contained in (domestic) sewage, especially nitrogen compounds.

2.2.1.3 Special wastewater disposal processes

In certain wastewater disposal areas, a combination of these two disposal systems may be appropriate. In some cases it may also be worthwhile in terms of ecology and water management to have a sewerage system in which only the sewage is centrally removed, but not the rainwater.

Moreover with the dual system, overground or underground removal of rainwater may also be a good idea in terms of ecology and water management if the sewage and rain water are consistently separated, taking care to ensure that the rainwater remains as "clean" as possible. In other words: The rainwater, which is relatively clean from the outset, should not be deliberately mixed with a medium (in this case sewage) whereby it likewise becomes a dirty medium. With careful and proper operation of such a dual system, the pollutant load in the receiving bodies of water can be substantially reduced, for the following reasons in particular:

- There is no need for rainwater or mixed water outfalls, therefore there are no mixed water effluents, which can otherwise cause serious pollution of receiving bodies of water, particularly after long dry spells.
- Only sewage passes to the central treatment works, so that the wastewater flow at the intake is considerably reduced and homogenized, thus considerably improving the efficiency and safety of the works.

2.2.2 Impact of wastewater treatment

2.2.2.1 Introductory notes

The qualitative and quantitative criteria for proper wastewater treatment - and so for its environmental impact - are derived primarily from emission and immission standards, which in turn are derived from the relevant water management conditions and from the legislation, regulations etc. in force. In many countries the latter rarely exist or where they do exist are inadequate. Direct application of, say, German, EC or American laws and regulations rarely provides an appropriate solution. Rather, it is necessary to develop measures suited to the prevailing general constraints and to implement them with the involvement of the local population.

2.2.2.2 Emissions from (central) sewage works

The substances in wastewater which pollute the water and sewage sludge in a public (municipal) wastewater disposal plant require a variety of processes and facilities to eliminate or reduce them. When planning a sewage works these are combined in a certain way or arranged in series (treatment stages). Table 1 summarises the technically feasible processes for treatment of municipal wastewater on the basis of the present level of development - with their treatment capacities expressed as the degree of efficiency (5).

The procedures in question are listed in the sequence in which they normally occur in the treatment stages of a sewage works, in order to achieve optimum results. Most importantly, the table shows the anticipated impact on the receiving body of water, i.e. pollutant emissions as a percentage of the concentrations in the incoming wastewater.

Table 1 - Efficiency of different wastewater treatment processes (expressed in %)

Process Suspended solids BOD51) COD2)

Phosphorus
Ptot

Nitrogen
(NH
4-N)

Mechanical treatment
Biological aerobic treatment

40 to 70
85 to 90

25 to 40
85 to 95

approx 15
approx 80
approx 15
approx 30
approx 7
approx 40

Additional reduction of residual substances

Micro-strainer
Filtration
Chemical precipitation
Activated carbon adsorption

20 to 40
50 to 80
70 to 90

50 to 90

5 to 10
10 to 20
50 to 85

approx 95

5 to 10
5 to 20
40 to 70

approx 90

-
-
50 to 90

approx 90

-
-
0 to 30

approx 10

1) Biochemical oxygen demand after 5 days
2) Chemical oxygen demand

For the efficiency of anaerobic wastewater treatment processes (highly suitable in countries with hot climates) please refer to (10).

Sewage works affect the environment not only in terms of water-related emissions, but also in terms of

- noise
- odours and
- air pollution (aerosols).

As a rule, however, it can be assumed that these types of emissions are less important than the water-related emissions of a sewage works (wastewater discharge).

2.2.2.3 Emissions of small sewage works

In the case of decentralised wastewater disposal systems individual or small sewage works may be involved (see 2.2.1.1). The disposal of the treated wastewater can either take place via discharge into a body of surface water or via discharge into the subsoil (seepage, percolation).

As regards permissible emissions of small sewage works and their environmental impact one must basically make a distinction between two types of works:

a) works without wastewater aeration, also called septic tanks (with exclusively mechanical or partly biological (anaerobic) treatment) and

b) works with wastewater aeration and mechanical/biological treatment. With works of type a), biological efficiency of 20-25% and in exceptional cases as much as 50% is obtained. In works of type b) efficiencies as high as those of central sewage works (see 2.2.2.2, Table 1) can be achieved, as long as they are properly designed and operated.

By adding an underground seepage plant, a sand filter pit or a soakaway, the wastewater in works of type a) can undergo further biological treatment and thus - provided the hydrogeological conditions are suitable -passed to the subsoil. The discharge of wastewater from works of type a) directly into surface waters is generally not acceptable.

2.2.2.4 Impact on bodies of water

Inadequately treated wastewater may disturb the natural self-purification capacity of receiving bodies of water based on physical, chemical and biological processes and cause other nuisances.

Undissolved wastewater components cause deposits of sludge, particularly in slow-flowing and standing waters, e.g. ponds, lakes and shipping canals, and also in blind river arms, creeks, reservoirs etc. If these originate from organic sediments, there will also be decomposition phenomena with the development of decomposition and fermentation gases, consumption of the oxygen dissolved in the water by absorbed decomposition products, inhibition of life forms or even mortality of microorganisms and fish. Many organically polluted commercial or industrial wastewaters favour the development of "sewage fungus" in flowing waters containing oxygen, particularly in cold seasons. Torn-off fungal particles develop into fungal drifts and, where the current is weak, frequently cause secondary sludge deposits with the results mentioned above.

Dissolved and organic constituents of wastewater require the presence of a certain quantity of oxygen in the water for their biochemical breakdown, where this occurs aerobically. This is determined in the same way as in the wastewaters themselves and is hence called biochemical oxygen demand.

Where the oxygen content or the oxygen uptake capacity of a receiving body of water is not sufficient for the biochemical oxidation of the organic substances fed into it, their further breakdown proceeds anaerobically. This results in the bacterial reduction of nitrates, sulphates, oxygen-containing organic compounds etc. to form carbon dioxide, hydrogen sulphide or sulphides, ammonia, nitrogen and other decomposition products. Methane is also formed in digested sludge (14). Anaerobic decomposition processes caused by oxygen deficits disrupt the largely aerobically structured self-purification capacity of a body of water, and in serious cases may even cause it to fail completely. Such problems can also occur in a body of water under certain conditions even if treated wastewater is discharged in the proper way, i.e. in accordance with the rules of the art. One must then however assume that the self-purification capacity of the body of water is insufficient with the given discharge conditions. In such cases more stringent standards must then be applied for wastewater discharges, in order to meet or restore the required water quality targets (see 3.3).

Besides the problems caused by lack of oxygen in biochemical oxidation, eutrophication, i.e. the accumulation of plant nutrients in the water, particularly phosphorus and nitrogen, is an important factor. This "over-fertilisation" causes a mass development of plant material, in particular water plants (foliage plants) as well as blue, green and filamentous algae. Controlled (aerobic) decomposition of the dying matter is then no longer possible and this results in the above-mentioned anaerobic water-polluting decomposition processes.

The water may be polluted by a number of other substances having a toxic effect on aquatic fauna besides phosphorus and nitrogen. These include heavy metals, volatile halogenated hydrocarbons (e.g. trichloroethylene), non-volatile halogenated hydrocarbons (e.g. chlorobenzols), dioxins, pesticides and polycyclical aromatic hydrocarbons (e.g. fluoranthene).

As aquatic fauna differ in their sensitivity to environmental burdens or pollution, they can be used as indices of contamination (bio-indicators). This is particularly true of burdens caused by lack of oxygen following the decomposition of organic matter and of toxic burdens. The saprobic system is based on this (7).

2.2.3 Impact of disposal of faecal matter

Much of what has been said in 2.2.1.1 ("adverse effects of decentralised wastewater disposal on the environment") apply here too. The following typical (adverse) effects of the disposal of faecal matter (in aerated/unaerated latrines, cesspools and collection pits) should be mentioned here:

- health hazards due to the use and the emptying of latrines and collection pits (risk of infection due to direct contact with the faecal matter; insect and rat infestation etc.)
- contamination of the subsoil, and particularly of the groundwater too, if the hydrogeological conditions are unfavourable
- health hazards upon final disposal (discharge) of the faecal matter, where not carried out properly
- aesthetic and odour nuisance.

Beneficial effects: See also 2.2.1.1 ("beneficial effects of decentralised wastewater disposal on the environment").

2.2.4 Impact of wastewater discharge

As stated in 1.4, wastewater discharge means the return of wastewater to the natural water cycle. This takes place in both disposal systems (decentralised and central).

Regarding the impact of wastewater discharge in the case of decentralised wastewater disposal, see 2.2.1.1 and 2.2.2.4.

Apart from likely noise and odour emissions, the effects of wastewater discharge in the case of central wastewater disposal manifest themselves mainly in the pollutant burden imposed on the receiving body of water, caused by the discharge from the central sewage works. Rainwater outfalls in a combined system can also have an impact on bodies of water. Furthermore, what is said in 2.2.2.4 also applies to the impact of wastewater discharge in the case of central wastewater disposal.

2.2.5 Impact of sludge disposal

2.2.5.1 Sludge disposal in central wastewater disposal

The sludge produced in a central sewage works must be treated in the course of disposal. The most important treatment stage is stabilisation; this may be carried out anaerobically or aerobically (7), (8). In the case of anaerobic treatment (digestion) of the sewage sludge, sludge digestion gases are produced which are largely odourless if the digestion process is carried out properly, i.e. through alkaline fermentation or methane fermentation (8). The main gases produced are carbon dioxide, nitrogen and methane.

Bearing in mind that recycling is required wherever possible, agricultural use of suitably pre-treated municipal sewage sludge should, if possible, be regarded as the correct disposal strategy for this sludge.

This should not however lead to accumulations of heavy metals in the soil, as these can pose a threat to people and animals via the food chain, particularly in the case of the highly toxic heavy metals cadmium and mercury.

If one considers the impact of sewage sludges in terms of their value as a source of raw material for agriculture, one should note (15):

- Sewage sludges are valuable primarily as phosphate and nitrogen fertilisers, but also because of their calcium and magnesium content; on the other hand, the potassium content is negligible. The organic matter content of sewage sludges also has a certain value. It is therefore only logical to use recycled sewage sludges for agricultural purposes.
- Sewage sludge which contains an excess of toxic components or which may have other adverse effects must not be used. Negative effects such as

damage to soil organisms and plants (phytotoxicity),

damage to the health of humans and animals as a result of excessive absorption through the food chain (via accumulations in plants);

adverse consequences for public hygiene

may be caused by an excess of potentially toxic elements.

- The plant availability of all components is a crucial factor. With agricultural recycling of sewage sludges the

phosphate content available to plants,

nitrogen content available to plants,

pollutant content available to plants,

are of prime importance. The last of these is determined by the content of seven potentially toxic non-ferrous metals (cadmium, chromium, copper, lead, mercury, nickel, zinc) in sewage sludges, as well as in the soils on which the sludge is to be spread.

Concerning the impact of sewage sludges used in agriculture, in conjunction with production of waste/sewage sludge compost, see also (16), (17), (18), (19).

2.2.5.2 Sludge disposal in the case of decentralised wastewater disposal

The sludge produced in decentralised disposal systems in small sewage works is mostly treated anaerobically. Where such a sewage works is operated correctly, no significant odour nuisance or hygiene problems should occur (20), (21). What is said in 2.2.5.1 applies here too, particularly as regards sludge disposal.

As the sludge to be removed from small sewage works is not always uniformly or sufficiently stabilised or sterilised (particularly in the case of mixtures of faecal matter and carrying water in wastewater collection pits), it may be a good idea to have secondary digestion carried out centrally, e.g. in open earth basins or tanks. This applies particularly where agricultural use is envisaged. Such a simple method of secondary sludge treatment is acceptable wherever the extra work involved and the sometimes unavoidable odour emissions are a comparatively minor problem.

2.3 Avoidance and safety measures

2.3.1 Wastewater avoidance

Wastewater which has not been produced does not need disposal! In other words, the use of appropriate procedures and measures to reduce the volume or avoid producing wastewater takes pressure off the capacities of wastewater disposal systems.

In the domestic sphere, wastewater can largely only be avoided through water-saving by the general public, e.g. through the installation and use of water-saving sanitary installations, etc. Such measures should not however be to the detriment of health and the proper collection and removal of wastewater. This also depends on individual citizens having the necessary motivation and understanding, which can be promoted by means of appropriate and regular information campaigns by the authorities and corporations responsible for wastewater disposal.

The positive effect of introducing progressive consumption tariffs on the water-saving behaviour of the general public should not be underestimated.

In the commercial and industrial sphere, depending on the sector from which the wastewater originates, specific plans should be developed to reduce the volume of wastewater. Such considerations usually centre on the recycling (multiple use) of process water, if necessary with the help of efficient treatment measures. Strict separation of sub-cycles is often highly advisable in this regard (14), (22).

2.3.2 Safety measures

2.3.2.1 Introductory notes

In this section, the term "safety measures" is used to denote all those measures which serve to minimise and compensate for the environmental impact and, where applicable, to make up for disturbances of the natural order.

2.3.2.2 Safety measures in wastewater collection and removal

In the design, construction and operation, primarily of central sewerage systems, but also of decentralised sewerage systems, the objectives should be as follows:

a) safe collection and removal of sewage and rainwater, not least in order to protect against disease
b) maintenance or improvement of the quality of surface water and groundwater
c) construction of permanently watertight sewers and repair of leaking sewers, pressure pipes and drains
d) optimisation of drainage works.

The above objectives can be achieved in particular by the following measures or procedures:

a)

- appropriate and adequate dimensioning of sewers and storage chambers to cope with peak flows (avoidance of flooding of properties, roads and land)
- suitable routing of sewers and arrangement of outfalls (in combined systems);
- flow control installations
- use of materials which fully meet the technical and hygiene standards.

b)

- reduction of discharge volumes (overflow frequency, discharge total, duration, load) at the outfalls of combined sewerage systems
- elimination of faulty connections in dual systems (with rainwater and sewage channels)
- reduction of volume of wastewater (rainwater, sewage and mixed water) e.g. by rainwater percolation, cooling and industrial water circuits in commercial and industrial establishments, reduction in water consumption (see 2.3.1)
- prevention of water inflows from ditches, springs, streams and drainage pipes (to be carried away only in exceptional cases and only in rainwater sewers in the dual system, with allowance for possible flooding).

c)

- use of high-grade components (particularly pipes) and sealing materials/sealing elements which behave well under long-term stress. This prevents, on the one hand, penetration of groundwater and percolation water into the sewerage network and, on the other hand, leakage of wastewater and its constituents into the subsoil, and hence into the groundwater.

d)

- provision of qualified and well-motivated personnel for monitoring, maintenance and servicing work
- provision of adequate resources (sufficient tariffs) to cover the costs incurred (23).

2.3.2.3 Safety measures in wastewater treatment

To avoid harmful pollution of the environment and particularly of surface waters, the following principles in particular should be observed:

- It is vital to determine as accurately as possible the composition and quantity of the wastewater produced and flowing to the sewage treatment works, particularly taking account of the short-term variations in domestic sewage quantities (daily maximum, daily minimum), quantities and constituents of commercial and industrial sewage (pre-treatment installations may be needed on the industrial sites in question) and the rainwater discharge conditions in the drainage area (7), (8)
- Reasonable allowance must be made for climatological conditions (level and distribution of annual rainfall, hours of sunshine, mean annual, monthly and daily temperatures etc.)
- The treatment capacity of the sewage treatment works must be appropriate to the ecologically acceptable and use-related load capacity of the receiving water system, paying close attention to the existing and anticipated preloading.
- All relevant technical and health regulations must be complied with when using treated wastewater and sewage sludge on agricultural land.

Where it is necessary to apply the technologically simplest wastewater treatment processes, even if the work requires more land and manpower, and particularly in countries with a hot and very sunny climate, aerobic wastewater oxidation ponds without artificial aeration are suitable (with or without a preceding anaerobic stage). They have proved to be a highly successful method of treatment (7), (8), (24), (25), (26), (27), (28). The operational and ecological advantages of these systems are:

- simple management; low maintenance and upkeep cost of system components
- discharge is highly suitable for irrigation purposes
- disinfection is quite adequate if the holding times required by the system are adhered to (total reduction of bacterial burden 97-98 % and more)
- low odour emissions under proper operating conditions and low volume of stabilised sludge.

Furthermore, with a view to safeguarding resources, particularly in countries with a hot climate, closer attention should be paid to processes utilising the valuable substances which the wastewater contains (wastewater utilisation). These include various digestion processes (biogas extraction), processes for agricultural exploitation (fertiliser production) after adequate desludging and fish pond processes (nutrient utilisation) (10), (29), (30), (31), (32).

Adverse effects occur mainly when the principles listed at the outset are not observed. In the case of oxidation ponds it is also worth mentioning that although these have a good buffer capacity to cope with sudden large quantities of wastewater, persistent operating problems are to be expected if wastewater with toxic constituents is delivered to the ponds, which in particular causes damage to the aerobic biosystem. Several weeks may pass before this is fully restored and the plant recovers the required treatment capacity, during which time the receiving body of water may be subjected to undesirable levels of pollution.

The following environmental protection measures can be taken to combat other non-water-related emissions:

- against noise: e.g. enclosure of motors and blowers.
- against emissions into the air: Covering of treatment basins; enclosure of treatment facilities such as automatic rakes, preaeration basins, etc. The waste air must be filtered (e.g. use of compost filter).
- treatment of the sewage sludge produced; aerobic stabilisation, anaerobic stabilisation (digestion), drying. Waste air or waste gas quantities produced must be filtered and heat-treated if necessary.

In addition, the sewage works may have to be landscaped in order to soften its visual impact.

2.3.2.4 Safety measures in sludge disposal

The sewage sludges produced during wastewater treatment in municipal main sewage works and in small domestic sewage works should - after treatment - be recycled in some suitable way. For example, they may be used to fertilise farm land. (see also 2.2.5). The same applies to the contents of cesspits, subject to adequate (secondary) treatment (see 2.2.5.2).

The composition of sewage sludges in terms of their content of heavy metals and non-degradable, sometimes toxic organic constituents is often a problem. This applies mainly to indirect discharges. Operators of public (central) wastewater disposal systems must then take particular care to ensure that commercial and industrial customers connected to the system discharge wastewater which is harmless both for the operation of the central sewage works and for the use of the sewage sludge on agricultural land (see also Section 3).

One should start from the principle that the sewage sludge is as "good" or as "bad" as the wastewater produced at the source. It is also important to monitor the indirectly discharged wastewater as carefully as the directly discharged wastewater, with particular emphasis on commercial and industrial producers.

It is vital for all community sewerage works initially to identify all commercial and industrial indirect dischargers, where necessary to demand suitable pre-treatment installations for the sites in question and thereafter, at least on a random basis, to monitor the discharge of the relevant plant.

It is also often a good idea to advise indirect dischargers on process-related wastewater management and wastewater avoidance and reduction, so as to avoid emission problems from the outset.


Contents - Previous - Next