The fieldwork could also be used as a secondary data source, .... treatment works on the water quality of an upland stre
Investigating the Effects of Treated Domestic Sewage on a Freshwater Ecosystem.
Discover Ltd. “Timbers”, Oxted Road, Godstone, Surrey. RH9 8AD www.discover.ltd.uk ©Discover Ltd 2007
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INVESTIGATING
THE EFFECTS TREATED DOMESTIC SEWAGE ON A FRESHWATER ECOSYSTEM .
TEACHER’S NOTES This unit investigates human impact on a freshwater ecosystem. A river is studied up stream and down stream of a small rural sewage treatment works, from which treated domestic sewage is discharged into the River Lot. Abiotic factors which may be affected by the presence of treated sewage in the river are tested: oxygen concentration and % oxygen saturation, biological oxygen demand (BOD), temperature, pH, dissolved and suspended load, and nitrates, phosphates and ammonium concentration. Students sample the stream fauna, to identify how these chemical changes affect the biotic community. The concept of indicator species is introduced, and how the presence of some species can give an indication of water quality, in the absence of abiotic data. A spreadsheet is used to analyse the biotic data, and a variety of statistical tests are carried out. Students are introduced to a range of concepts important for their understanding of human impact on freshwater environments, and to a number of techniques, which could be incorporated into coursework. The fieldwork could also be used as a secondary data source, or provide a contrasting freshwater environment to one studied in the UK.
KEY SPECIFICATION AREAS: •
Describe the ecological impact of human activity on the environment, to include water pollution and the effect of organic effluent, nitrates and phosphates on water quality, oxygen content and biodiversity;
•
Define the term indicator species and describe how such species can be used to assess practically the levels of water pollution in a given area;
•
Describe the sampling of water and assessment of biological oxygen demand (BOD), and explain how the technique can be used to monitor water quality;
•
Understanding the concept of diversity in the context of ecological stability, and calculating a diversity index where:
D =
N(N–1) Σn(n–1)
•
Calculating a biotic index.
BIBLIOGRAPHY Giller, S. G. and Malmqvist, B. (1998). The Biology of Streams and Rivers. Oxford. Huggett, R. J. (1998). Fundamentals of Biogeography. Routledge.
Mason, C. F. (1998). Third Edition. Biology of Freshwater Pollution. Longman.
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INTRODUCTION GENERAL INFORMATION Occasionally, natural events such as flooding and mudflows effect water quality, but most serious and chronic water pollution is caused by human activity. Pollution of water by humans means the deliberate or inadvertent introduction of substances or energy into waterways, which have deleterious effects. These substances may be found naturally in the environment in low concentrations, but when present in high concentrations, cause problems – such as high levels of nitrates causing eutrophication. Other substances may be completely synthetic, such as organic pesticides. The effects of pollutants vary depending on the capacity of the environment to remove them. In fact, the effects of treated organic effluent can be short-lived provided the receiving waters are high volume and relatively fast flowing. Some chemicals are lethal in minute amounts – the chemical tri-butyl tin (painted onto boats as an anti-foulant) is lethal at a concentration of a few parts per billion. Freshwater is a limited resource on earth. Only 3% of the world’s water is freshwater, the remainder is salt or brackish. Of this, 75% is locked up in the earth’s ice caps and so highly inaccessible, 24% is locked in aquifers in porous rocks and only 1% is present in lakes and rivers. Of this tiny amount, much is concentrated in small areas – the Great Lakes of North America, the Caspian Sea and Lake Bical. For the majority of people around the world, acquiring life’s basic resource is difficult. Water is needed: •
In the home for drinking, cooking and washing;
•
In industry for power generation, as a coolant, in food manufacturing, brewing and distilling;
•
In agriculture for irrigation and livestock;
•
As a medium for waste removal;
•
For it’s amenity value;
•
As a habitat for thousands of species of plants, invertebrates, birds and mammals.
As a result of this use, water is often contaminated. Sewage treatment and intensive agricultural land management practises are major causes of freshwater pollution. Sewage Treatment: Domestic sewage is rich in ammonium from urine and faecal matter, phosphates from detergents, and particulate matter. By proportion, it is approximately 45% carbohydrate, 45% fat and 10% protein. It may also contain coliform bacteria and intestinal parasites. The
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purpose of sewage treatment is the oxidation of toxic ammonium from protein deamination, to nitrate ions, by nitrification, before its release into receiving waters. The sewage treatment process involves screening to remove large items which may clog the process, such as nappies, tree branches (as water may also come from roads in towns) and grit. The sewage is then held in tanks to settle out and separate the liquid and solid portions. The solid effluent in then pressed and buried, incinerated or spread onto agricultural land. The liquid effluent continues onto the biological treatment stage in which aerobic bacteria carry out oxidation reactions, converting the ammonium ions to nitrate: 2NH4+ 2NO2-
+ +
3O2 O2
→ →
2NO22NO3-
+ 2H2O + H2O + 4H+(carried out by nitrosomonas) (carried out by nitrobacter)
Nitrosomonas and nitrobacter occur naturally in watercourses and if the sewage is released prematurely, or the treatment process is inefficient, the ammonia will be converted to nitrate and cause oxygen depletion in the receiving water. Because of this, treated domestic sewage must meet an EU standard, set to limit the effects of treated sewage on the environment, the 30 : 20 standard. Treated domestic sewage must have a BOD of less than 20mg/l and contain less than 30mg/l of suspended material. Domestic sewage is also rich in phosphates from ‘soft’ detergents, used in preference to the ‘hard’ detergents from the 1950’s and 1960’s, which contained long-chain carbon molecules, which did not break down in water and caused foaming in watercourses. ‘Soft’ detergents break down in water more easily, but their high phosphate content can cause eutrophication in slow-moving watercourses. Domestic sewage may also affect the temperature of the receiving waters. During the settlement process, the sewage is pooled for 12 – 24 hours, which will warm up during the summer months. Also, bacterial activity during the biological treatment process will increase the temperature of the final effluent. The pH of the final effluent will be slightly acidic, due to the release of hygrogen ions when ammonium is oxidised to nitrite by nitrosomonas. This will increase the conversion of the chemically inert ammonia into toxic ammonium. The effluent will contain a high amount of suspended matter (largely faecal material) from the treatment process, and a large amount of material in solution. This will include ammonium,
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nitrates and phosphates, but will also include iron, calcium, fluoride and some heavy metal ions such as lead, which will have been washed from roads in storm water run-off. Agricultural Contamination. Inorganic plant nutrients, silage, slurry and agrochemicals: Intensively managed agricultural land can give rise to groundwater and surface run off contaminated with inorganic fertilisers, slurry, silage leachate, and pesticides, herbicides and fungicides. Water courses on or near farm property can become contaminated by this diffuse pollution source. Inorganic fertilisers containing nitrates and phosphates enhance primary production in watercourses and can lead to eutrophication. Filamentous algae and some water macrophytes proliferate. The increased level of organic detritus leads to oxygen depletion due to increased numbers of decomposing bacteria in sediments. This effect is generally confined to slow moving or static watercourses. Eutrophication can have natural causes – on the Norfolk broads in the 1960’s, high numbers of over wintering sea birds caused eutrophication of many inland waterways. Eutrophication results in a loss of biodiversity and a reduction in the amenity value of watercourses. In oxygen-depleted water, bacteria causing botulism and Weil’s disease thrive, and blue-green bacteria may proliferate. High levels of nitrates in drinking water have associated human health implications – known to lead to increase risk of stomach cancer, and the infant disease, blue baby syndrome, in which nitrites compete for the uptake of oxygen in foetal blood. Due to these risks, the EU places a limit of 50mg/l on nitrates in drinking water. Agriculture can also inadvertently introduce slurry and silage leachate into watercourses. Both are rich in ammonium (especially if the local geology is acidic, as ammonia is reduced to ammonium ions under acid conditions). Ammonium ions are nitrified first to nitrite ions (NO2-) by nitrosomonas bacteria, then to nitrate ions (NO3-) by nitrobacter bacteria. These oxidations deplete oxygen. Pesticides, herbicides and fungicides may enter watercourses through ground water, surface run off or through the air. Many of these are based around a heavy metal ion such as mercury, lead, aluminium, cadmium and zinc. They can be biomagnified through the food chain to toxic levels. Some heavy metals have synergistic effects such as copper and cadmium, which are far more toxic when they occur together than when they occur singly. Some heavy metals may behave antagonistically, such as zinc and cadmium, which when found together reduce the toxicity. All heavy metals are more toxic in the ionic state which arises under acidic conditions. Other agrochemicals are organically-based and bioaccumulate in the fatty tissues of organisms, biomagnifying up the food web, before having lethal or sub-lethal (affecting reproductive success) effects. Such chemicals include DDT, dieldrin and PCB’s.
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SPECIFIC INFORMATION The sewage treatment works in St. Jean du Bleymard serves two settlements – St. Jean du Bleymard and Le Bleymard – with a combined population of 446 (1999 population census data) people. It also receives the waste from a small holiday village in a neighbouring valley, when it is open in July and August. The total population may then rise to 1200 people. The sewage treatment works has a simple screen and settlement tank. Biological treatment is carried out in a large open tank agitated at five-minute intervals by a large propeller. The sold waste is stored in several large tanks close by and is collected during the autumn and winter by local farmers who use it as an organic fertiliser. The liquid waste is discharged almost continuously in low volumes, and diluted approximately 1:1000 by the river Lot at discharge.
AIMS •
To determine the effects of point source organic pollution from a small rural sewage treatment works on the water quality of an upland stream, and study the recovery of the stream;
•
To investigate the effects of water quality changes on stream fauna.
OBJECTIVES •
To determine how treated sewage affects oxygen concentration, % saturation and BOD, temperature and pH, ammonium, nitrates and phosphates levels, suspended and dissolved load, of the receiving stream by testing water quality above and below a sewage out fall;
•
To determine the susceptibility of freshwater invertebrates to organic pollution, and to use indicator species to give an indication of the extent of chronic (long term) organic pollution in the stream;
•
To determine the effects of organic pollution on species diversity and abundance, and to calculate the Simpson’s Diversity Index.
HYPOTHESES •
In stream water receiving treated sewage, the oxygen concentration will reduce; the % oxygen saturation will decrease; the water pH will become more acidic; the temperature will increase; the nitrate concentration will increase; the ammonium concentration will increase; the phosphate concentration will increase; the suspended and dissolved load will increase;
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•
There will be a change in the stream community, as pollution-tolerant species replace pollution-intolerant species;
•
With increasing distance from the outfall, the abiotic environment will return to normal control parameters;
•
With increasing distance from the outfall, pollution-intolerant species will return and replace pollution-tolerant species.
Students should be encouraged to think about how these abiotic factors are likely to change with increased distance from the outfall. These abiotic changes are not permanent, and the stream will ‘recover’ downstream of a discharge.
DATA COLLECTION SITES Four data collection sites in St. Jean du Bleymard are suggested (see Appendix 1): (1)
Control site: 20m upstream of sewage out fall pipe;
(2)
Outfall: directly at the sewage outlet pipe;
(3)
Dilution site: 5m below out fall;
(4)
Recovery site: 20m below outfall.
EQUIPMENT •
IN THE FIELD
Per group:
D-ring sampling net Small white collection tray Spoons Identification key Collection pot for counting animals Magnifying glass or hand lens
For whole class:Oxygen meter with integral thermometer Nitrate meter and indicator strips Narrow range pH indicator papers Conductivity meter 3 plastic beakers 8 water sample pots •
IN THE CLASSROOM
For demonstration:
Ammonium test kit
Safety glasses
Phosphate test kit
Safety gloves
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METHOD AND ORGANISATION OF STUDY As group data will be collated in the follow up, it is vital that data collection is standardised between the groups and is consistent between the four sites. Split the class into groups of three. If the class is large (>15), half of the groups sample sites 1 and 2, the other half samples sites 3 and 4. Abiotic data collection: NB. This should be done before the biotic data collection. Sampling should be carried out from the river bank, to avoid disturbing the riverbed, which affects suspended load and disturbs the invertebrates. (1)
Advise students about the potential hazard of Weil’s disease (speak to your group leader). Students must cover cuts with micropore tape and not splash each other. Students with eczema should wear latex gloves, or avoid contact with the water.
(2)
Divide students into work-groups and distribute abiotic equipment. Your group leader will demonstrate the use the equipment to the students;
(3)
Collect three sample bottles of water from each site in water containers;
(4)
Each group of students takes one reading with the piece of equipment they have been given, records the results on recording sheet 1, and then passes it to the next work team. Pass equipment between the sites as necessary.
Biotic data collection: The Environment Agency standardise data collection throughout Britain by using a three minute kick sample. It has been shown that this method provides the highest proportion of species present for the least time actually spent sampling. The three-minute sample should be divided equally between the groups – eg. Three groups of students should carry out a 1minute kick sample each. It is essential that all of the microhabitats in the stream be sampled within this time, such as in fine sediment, amongst larger pebbles, in weed and where leaves have accumulated. (1)
Distribute biotic equipment between work groups. Your group leader will demonstrate the kick sampling technique and the use of the key;
(2)
Tell students how long they need to kick sample for (3 minutes, divided by the number of work groups), and point out the boundaries for the sampling and any microhabitats to be included;
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(3)
Students kick sample carefully for the allocated time. Empty invertebrates into a white sampling tray, two-thirds full of stream water. Identify the invertebrates using the key, count into the pot, and enter onto tally list (recording sheet 2);
(4)
Carefully replace animals in the stream and total up invertebrate tallies.
CLASSROOM FOLLOW-UP Ammonium and phosphates concentration: Students must wear safety goggles and latex gloves for this work, and read the hazard information on the instructions. Simple ‘Aquamerck’ chemical titrations are used to establish ammonium and phosphates concentration in mg/l. Instructions are included in the kits – speak to your group leader. Allow up to one hour, depending on the number of samples. If you are not confident with the ability of yourself or the students to use these kits safely, ask your group leader to carry out this classroom work as a demonstration. Suspended Load: There are two possible techniques for this: 1.
Filter a litre of water from each site onto weighed filter paper. Dry to constant mass in the microwave oven and calculate the amount of suspended material in the sample. Remember, this should be less than 30 mg/l. Allow at least one hour for the filtration and half and hour for the drying process.
2.
Pour equal volumes of the samples into clear plastic lidded pots. Shake them and hold a piece of white paper behind them. Make a visual comparison – which sample contains most suspended material and which the least.
Biological Oxygen Demand (BOD): The initial oxygen concentration is determined in the field. The sample should then be kept in the dark, at approximately 20oC, for five days. The oxygen content in mg/l should then be reassessed. There must be a decrease in the oxygen concentration of no more than 20mg/l. If time at the centre does not allow leaving the sample for five days, either establish the oxygen decrease over a shorter time period, or take the sample back to school for examination at a later date.
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RECORDING SHEET 1 - FRESHWATER POLLUTION (ABIOTIC FACTORS)
Abiotic
Site 1
Site 2
Site 3
Site 4
Variable
(Control)
(Outfall)
(5m downstream)
(20m downstream)
O2 (% saturation)
Temperature (◦ C)
pH
Total dissolved load (mg/l)
Suspended load (mg/l)
Nitrates (mg/l)
Ammonium (mg/l)
Phosphates (mg/l)
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RECORDING SHEET 2 - FRESHWATER POLLUTION (INVERTEBRATE TALLY SHEET) Species
BMWP Score
Freshwater mites
4
Alderfly larvae
4
Stonefly nymph
10
Olive Mayfly nymph
4
Flattened mayfly nymph
10
Striped mayfly nymph
10
Cased caddis fly larva (small stones) Cased caddis fly larva (fine sand) Cased caddis fly larva (stones and debris) Cased caddis fly larva (vegetation only) Caseless caddis fly larva (brown) Caseless caddis fly larva (green)
10 10 8 10 6 8
Dragonfly larva
8
Elmid beetle and larva
5
Small diving beetle and larva
5
Cranefly larva (Tipula)
5
Cranefly larva (Dicranota)
5
Flatworms
5
Blackfly larva
5
Tubifex worm
1
Midge larva (green, brown and transparent)
2
Midge larva (red)
2
Leeches
3
TOTAL BMWP
Tally
11 TOTAL ABUNDANCE (N)
Tally Total
NAME……………………………..
SITE………………………………..
RECORDING SHEET 3 - FRESHWATER POLLUTION (BIOTIC FACTORS)
Biotic
Site 1
Site 2
Site 3
Site 4
Variable
(Control)
(Outfall)
(5m downstream)
(20m downstream)
(1) Total number of individuals found (N)
(2) Total number of Species found
(3) Dominant species
(4) Simpson’s Diversity Index
(5) Total BMWP score
(6) Mean BMWP score (5) / (2)
RECORDING SHEET 4 - FRESHWATER POLLUTION (SIMPSON’S DIVERSITY INDEX)
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Species
Tally total (n)
(n – 1)
n (n - 1)
N
∑ n(n-1)
D = N(N-1)
Freshwater mites Alderfly larva Stonefly nymph Olive mayfly nymph Flattened mayfly nymph Striped mayfly nymph Cased caddis fly larva (small stones) Cased caddis fly larva (fine sand) Cased caddis fly larva (stones and debris) Cased caddis fly larva (vegetation only) Caseless caddis (brown) Caseless caddis (green) Dragonfly larva Elmid beetle and larva Small diving beetle and larva Cranefly larva (tipula) Cranefly larva (dicranota) Flatowrms Blackfly larva Tubifex worm Midge larva (green, brown and transparent) Midge larva (red) Leeches
Totals
∑ n(n-1)
DATA PRESENTATION AND ANALYSIS Each group enters their abiotic and biotic data onto the spreadsheet on the computer in the classroom. This could be done during a break – allocating each group a time to report to the class room – or while students are writing up their methods. Calculate the means for each abiotic variable. Total up each invertebrate species. Print a copy of the master results and photocopy enough for each student. For the combined data for each site, allocate groups of students to calculate:
•
Species richness (the total number of species present at the site);
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•
Total abundance (the total number of organisms present at each site);
•
Dominant species (which species is most common);
•
Simpson’s Diversity Index (see equation, page 2);
•
Biological Monitoring Working Party Score (BMWP) biotic index (appendix 2).
Abiotic factors could be graphed as line graphs or histograms showing changes in water quality between the control site, past the out fall to the recovery site.
CRITICAL APPRAISAL How could the reliability of the results have been compromised by the data collection techniques? Students should be encouraged to critically appraise the sampling procedure and make suggestions for its improvement. Biotic sampling:
Were any invertebrates under sampled, such as those clinging to weed or rocks? Were invertebrates swept down stream due to high water levels or poor kick sampling technique? Was account taken of the variety of microhabitats in the stream channel?
Abiotic sampling:
Was there any possibility of contamination? Were samples taken from the out fall when the propeller was spinning?
If students plan to use this work as secondary data, or as part of a project comparing the Lot with a river at home, it is important to discuss the limitations of this idea. For example, time of year, species present, effect of local geology, etc.
DATA INTERPRETATION AND DISCUSSION During the discussion, the following points should be covered: •
How does treated sewage waste affect the quality of receiving water?
•
Why is it necessary to test water quality and look at indicator species – which of these two gives the most information about the long term ‘health’ of the river?
•
How long before water quality returns to normal?
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•
How does the river recover - how is the water re-oxygenated? What happens to ammonium, nitrates and phosphates downstream from the outfall? Relate oxygen concentration, ammonium and nitrate levels to the oxygen sag curve in appendix 3;
•
How does the change in water quality affect the biotic community – total abundance, species richness (diversity), dominant species, diversity index, biotic index;
•
Do the invertebrates present reflect the water quality at each site – why are freshwater invertebrates useful as indicator species? (show both chronic and episodic pollution which an investigation into water chemistry alone would not reveal);
•
How does the biotic community changes below the out fall?
•
What is the significance of the diversity index score?
•
What is the significance of the biotic index score – what does the final score mean in terms of water quality (see note at bottom of appendix 3).
POSSIBLE PROJECT TITLES The following are some ideas for projects which students could undertake for coursework, using data collected in this unit. •
What are the effects of treated sewage discharged in to the River Lot?
•
How does water quality change downstream from a small sewage treatment works?
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•
How can biological indicators be used to show the effects of discharge from a small rural sewage treatment works?
•
Are biological indicators alone, sufficient to show water quality change?
•
What is the relationship between oxygen concentration, and ammonium and nitrate concentration upstream and downstream of a sewage treatment works?
•
How does the discharge from a sewage treatment works affect the biological community of a small upland stream?
•
What is the rate of recovery of the river Lot, following the input of treated domestic sewage?
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APPENDIX 1 - Map of study sites.
To Mende
Sewage Treatment Works 4 St Jean du Bleymard
3 2 1
D901 To Villefort
River Lot Site 1 – 20m upstream of outfall (control). Site 2 – Outfall Site 3 – 5m below outfall. Le Bleymard Ste 4 – 20m below outfall Holiday Village
To Eagle’s Nest
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APPENDIX 2 - CALCULATING THE BMWP SCORE Species / Family Mayfly nymphs (flattened and striped only) Stonefly nymphs (large and small) Cased caddis fly larvae (cases made from stones/sand only) Cased caddis fly larvae (cases made from vegetation only) Caseless caddis fly larvae (green) Dragon fly nymphs Damsel fly nymphs Cased caddis fly larvae (cases made from vegetation and stones) Freshwater shrimp Freshwater limpet Caseless caddis fly larvae (brown) Freshwater mussel Water boatman All water beetles and their larvae Water measurers Pond skaters Black fly larvae and pupae Cranefly larvae (tipula and dicranota) Flatworms Mayfly nymph (olive) Alderfly larvae Water mite Fish leech Freshwater cockle Fresh water snails Leeches (except fish leech) Water hoglouse Midge larvae (green, brown and transparent) Red midge larvae (‘bloodworms’) Freshwater worms (including tubifex)
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Score 10
8
6
5
4
3
2 1
BMWP score 151 + 100 – 150 51 – 99 16 – 50 0 - 15 APPENDIX 3
Environment Agency (EA) Water Quality grade EA River Ecosystems Grade Very clean RE1 Clean RE2 Intermediate RE3 Polluted RE4 Very polluted RE5 - Relationship between oxygen concentration and ammonium and nitrate
concentration below a sewage discharge.
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Outfal l
BOD5
A Oxygen
Suspended Solids
NH4 B
NO3 PO4
20
