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4 Organic pollutionOrganic pollutants can be divided into following categories:

a) Oxygen Demanding wastes:b) Oilc) Other Organic Compounds polyaromatic hydrocarbons, organic pesticides, polychlorinated biphenyls (PCBs) dioxins.

Characteristics of organic pollutantsMany of these compounds are toxic, this toxicity can be acute or chronic highly persistent and lipophilic, which means that they are amenable to bioaccumulation.

4.1 Oxygen Demanding wastes:The wastewaters such as, domestic and municipal sewage, wastewater from foodprocessing industries, canning industries, slaughter houses, paper and pulp mills,tanneries, breweries, etc. have considerable concentration of biodegradable organiccompounds either in suspended, colloidal or dissolved form.

Oxygen DemandOrganic wastes discharged into water courses, estuaries and the seas are subjected tobacterial degradation, which result in the oxidation of organic molecules to stableinorganic compounds. In this process oxygen is consumed and the bacterialpopulation increases.Aerobic bacteria use the oxygen dissolved in the water to achieve this

C6H12O6 + 6O2 →6 H2O + 6CO2(glucose) (oxygen) (water) (carbon dioxide)

As the result of this bacterial activity, the oxygen concentration in the water isreduced, but this is compensated for by the uptake of atmospheric oxygen. However,oxygen diffuses only slowly through the water and there is a time-lag before theoxygen used by bacterial activity is replenished.• Depletion of the dissolved oxygen (DO) will be a serious problem

o If the DO falls below 4.0 mg/L this will adversely affecting aquatic life,.This decrease of DO is an index of pollution.

o If the oxygen concentration falls below 1.5 mg l-1, the rate of aerobicoxidation is reduced. Anaerobic bacteria can oxidize organic moleculeswithout the use of oxygen, but the end-products include compounds such asH2S (hydrogen sulphide), NH4 (ammonia) and CH4 (methane), are toxic tomany organisms, and this process is much slower than aerobic degradation.

• Some inorganic wastes become oxidized in water without the intervention ofbacteria and these, too, deplete the water oxygen.

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Measurement of oxygen demand

Chemical oxygen demand (COD)Chemical Oxygen Demand or COD is a measurement of the oxygen required tooxidize soluble and particulate organic matter in water. Accordingly it is indirectmeasurement of the amount of organic pollution in a sample of water. It is expressedas parts per million (ppm) or milligrams per liter (mg/L), which indicates the mass ofoxygen consumed per liter of water.How is COD measured? A common method for Chemical Oxygen Demand analysisinvolves using a strong oxidizing chemical, such as potassium permanganate(KMNO4) or potassium dichromate (K2Cr2O4), to oxidize the organic matter insolution to carbon dioxide and water under acidic conditions. The sample is thendigested for approximately 2 hours at 150°C. The amount of oxygen required iscalculated from the quantity of chemical oxidant consumed. The higher the chemicaloxygen demand, the higher the amount of pollution in the test sample.

Biochemical oxygen demand (BOD)What is BOD? It is the amount of dissolved oxygen consumed by aerobic biologicalorganisms (mainly bacteria) to break down organic material present in a given watersample at certain temperature over a specific time period.How is BOD measured? The most common BOD test consists of a 3 or 5 daysperiod (BOD3 or BOD5 respectively) in which a sample is placed in an airtight bottleunder controlled conditions temperature (20ºC ± 1ºC), keeping any light frompenetrating the sample to prevent photosynthesis. The Dissolved Oxygen (DO) in thesample is measured before and after the 5 day incubation period, and BOD is thencalculated as the difference between initial and final DO measurements.

The dilution factorIf water is saturated with oxygen there is sufficient oxygen to oxidize a BOD5 ofabout 8.0 – 8.5 mg l-1, but the BOD5 of organic effluents is usually much greater thanthis. Urban sewage commonly has a BOD5 of 500 mg l

-1, it is therefore necessary tofind some way of diluting the effluent to achieve a BOD5 of about 8 mg l

-1, hence theold adage of sanitary engineers: 'the solution to pollution is dilution'.The common practice is to discharge the effluent into a large volume of water,preferably a river or the sea, where water movement produces mixing and achievesthe necessary dilution.If the effluent is discharged into a river (Fig. 4-1), it is swept downstream and mixeswith the river water in a mixing zone. The concentration of effluent is high near theoutfall, but progressively reduced downstream. The dilution achieved depends upon(1) the rate of flow of the river, (2) its organic load, and (3) the rate of input of theeffluent and (4) its oxygen demand.

Fig. 4-1 Discharge of an organic effluent into a river and its mixing zone.

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For example,River flow 8 m3s-1 with BOD 2 mg l-1

Effluent input 1 m3s-1 with BOD 20 mg l-1

BOD after mixing =total BOD

total volume

=(8 × 2) + (1 × 20)

48 + 1

= mg l-1

This achieves the necessary dilution.

Expected consequences of organic discharges

EnrichmentDecaying organic matter, and nitrate and phosphates in sewage enhance the growth ofphytoplankton and fixed plants, and this enrichment benefits a number of food chains,as do the increased bacterial populations stimulated by an organic input. A moderateinput of nutrients has the same effect in the sea as adding fertilizer or manure togarden or farmland. The elevated input of nutrients in the Seto Sea in Japan forexample boosts the primary production. As a consequence, fishery catches in the seahave increased in parallel with the rise primary production.

EutrophicationAlthough a moderate input of organic material may be beneficial, over-fertilizationresult in extravagant growth of plants and bacterial decay of dead plant material mayresult in oxygen depletion. Both excessive plant growth and oxygen depletion lead toalternation of the community structure, sometimes disastrously. These phenomena arefeatures of eutrophication and are a familiar problem in freshwaters; it is only recentlythat this has been recognized as a problem in the sea. Chlorophyll-a concentrationabove 0.5 mg m-3, are thought to be a threshold for marine eutrophication.The problem associated with eutrophication are very varied.• A common sign of sewage pollution on the shore is the growth of green algae,

such as Enteromorpha and Ulva, and in some areas these form a dense covering ofintertidal sand or mudflat.

• Eutrophication is responsible for damage to coral reefs. Increased algal growthreduces light intensity, affecting photosynthesis by the coral's zooxanthellae, andit increases sedimentation, smothering the corals.

Algal bloomsA large input of plant nutrients often results in the development of red tides. Manyanimals, including commercially important fish species, are killed or excluded fromthe area, either of clogging of the gills or other structures, or because of the toxicproperties of the phytoplankton organisms.

Oxygen depletionAnother damaging effect of over-enrichment is that the dense blooms ofphytoplankton result in a very high concentration in the surface water because ofincreased photosynthesis. The abundance of decaying plant material falling in theseabed severely reduces the oxygen concentration in bottom waters, and most benthicanimals are killed or excluded from the area. Deoxygenation of bottom waters hasbeen recorded for many sea areas.

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Unexpected consequences of organic dischargesAttention has focused chiefly on the harmful effects of adding organic wastes to themarine environment. Reduction or elimination of discharges of organic wasteshowever, is not invariably beneficial.• Fisheries in the Japanese Seto Sea benefited dramatically with the increase in

organic discharges (Fig. 4-2).

Fig. 4-2 Relationship between fish catch and primary production between 1951 and 1980 inthe Seto Sea, Japan

• Tubifex worms flourish in conditions of high organic input and low oxygenconcentration. These worms are a major food source for seabird . With thereduction of the pollution load, the benthic biomassand the number of birds feeding on it fell.

• Sewage sludge dumping resulted in localenrichment and the growth of algae on intertidalrocky shores. The algae supports a large populationof grazing gastropods that, in turn, provided foodfor a large seabird population. With the cessationof sludge dumping the algae, gastropods, andnumbers of shore-feeding birds have declined.

• Number of sea ducks declines as Thames estuary (UK) restored.

Sewage PollutionPublic health risks

Beach may be closed temporarily due tosewage contamination. Contamination ofbathing water (or seafood) may pose ahealth hazard on the human population.Swimming–related illnesses caused bypathogens (disease-causing organisms)include sore throats and diarrhea;respiratory, ear, eye and skin infections;and more serious illnesses like meningitisand hepatitis.

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Pathogens in sewageAll human sewage contains

• Enteric bacteria, e.g., Salmonella, Shigella• Virus, e.g., polio, hepatitis virus, rotavirus• Protozoa, e.g., Giardia, Cryptosporidium• Eggs of intestine parasites e.g., Ascaris

Routes of infection:• Through contacts with pathogenic bacteria,

viruses or yeasts• Through accidental ingestion of pathogens• Through infection by parasites present in

the sewage or sewage sludge• Through consumption of contaminated

seafood

Survival of pathogen in seawaterFormerly, it was believed that pathogenic bacteria and viruses didn’t survive inseawater and that sea bathers were at little risk from sewage contamination unless thepollution was so gross as to be visible. This is not true, where;

• Bacteria, and Virus can be very persistent in seawater, thus, cuts and skinabrasions may become infected, and swallowing of seawater may result inacute diarrhea caused by bacteria Salmonella or Shigella, or infection by polioor hepatitis viruses.

• Parasites’ eggs (such as Ascaris) may complete their life cycle in sea and leadto infection of human.

SeafoodThe chief health risk from sewage discharge to sea is undoubtedly through theingestion of contaminated seafood

• Higher risk for filter feeders, e.g., bivalve mollusks which accumulate humanpathogens on their gills and these may be transmitted to the consumer.

• The risk is greatest if the bivalves are freshly harvested and eaten uncooked.• Depuration (transferring to uncontaminated water for a few weeks) is required

before marketing to be safe to eat.• Crustaceans and fish do not presents risks as they do not accumulate

pathogens from sewage contaminated water.

Biotoxin:• Produced by harmful algae• These include Ciguatera, paralytic shelfish poisoning (saxitoxin) and

neurotoxic shellfish poisoning• Most such cases are unrelated to pollution, but the incidence of blooms of

toxic dinoflagellates has increased in recent years, with serious public healthconsequences, and it is suspected that nutrient enrichment of the waters hasbeen a contributory factor.

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Sewage treatmentSewage has a high oxygen demand. This can be reduced by treatment. The sewagetreatment involves three stages, called primary, secondary and tertiary treatment.

• In primary (mechanical) treatment, the solids are separated from thewastewater stream. Primary treatment can reduce the BOD of the incomingwastewater by 20-30% and the total suspended solids by some 50-60%.

• In secondary (biological) treatment dissolved biological matter isprogressively converted into a solid mass by using microorganisms. About85% of the suspended solids and BOD can be removed after settling by a wellrunning plant with secondary treatment.

• The tertiary treatment, is simply additional treatment beyond secondary!Tertiary treatment can remove more than 99 percent of all the impurities fromsewage, producing an effluent of almost drinking-water quality. The finaleffluent can be discharged into a sea, stream, river, bay, lagoon or wetland, orit can be used for the irrigation of a golf course, green way or park.

The various treatment processes may reduce: Suspended solids, Biodegradableorganics (e.g. BOD), Pathogenic bacteria and other disease causing organisms andNutrients, including nitrates and phosphates.Remaining solids (sewage sludge) with a high BOD is used as an agriculturalfertilizer, transferred to landfill sites, or dumped at sea.

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Effect of sewage and sewage sludge dumping on benthic organisms• Smothering by the particulate matter• reduction of the oxygen concentration because of enhanced bacterial activity• excludes the more sensitive species, but the more tolerant species flourish

because of the input of extra nutrients to the system. The results is a reductionin diversity but an increase in the abundance of organisms (Fig. 4-3)

Fig. 4-3 Effect of sewage on abundance and diversity of benthic fauna.

Monitoring Water Quality at Bathing SitesThe count of faecal coliform (FC, Gram-negative, rod-shaped bacteria) and faecalstreptococci (FS, Gram-positive coccoid bacteria) in seawater samples can be used toassess the seawater quality on the basis of compliance with microbiological standards(Table 4.1) set out by European Bathing Waters Directive (76/160/ EEC). The countis a good measure of the risk to which human population is exposed. These bacteriaare always present in the intestine of human and other warm blooded animals andappear in their faeces.

Table 4.1 Mandatory and guideline standards for fecal coliform and fecal streptococci set byEuropean Union Bathing Water Directive 76/160/EE.

Organism Mandatorya, b Guidelinea, b

Fecal coliform (FC) 95% < 2000 80% < 100

Fecal streptococci (FS) No level set 90% < 100a Colony forming units (CFU) 100mL-1.b Minimum percentage of 20 samples required to meet standard in order to achieve compliance.

The Directive sets two levels of compliance;"mandatory" إلزامي which must be achieved, and"guideline" توجیھي which should be aimed for.The guideline standards are twenty times more stringent than the mandatory.

Procedure:• Monitoring points are established where most bathers are expected and where

there is the greatest risk from pollution.• Water quality monitoring at these sites must take place throughout the bathing

season, (15th May to 30th September) with twenty weekly samples collected.

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• Substances tested for are:o Microbiological parameters: total and faecal coliforms and faecal

streptococci.o Physico-chemical parameters: surface active substances, mineral oils

and phenols.

The mandatory standards are 2,000 cfu/100ml fecal coliform, 95% of samplesshould comply. Of the twenty samples taken in the season this allows one sample toexceed and the site to still pass (the one sample represent 1/20 = 5%, the remaining 19represents 95% of the total samples).Guideline standards are 100 cfu/100ml fecal coliform and fecal streptococci. 80% offecal coliform samples must comply and 90% of fecal streptococci samples shouldcomply. This equates to four exceedances for faecal coliforms and two for faecalstreptococci in a season. The results can be assessed to give an overall category of"guideline" pass, "mandatory" pass, or "fail" at the monitored sites.

Issues in seawater and beach quality of Gaza stripGaza beach is considered the main recreational site for a population of more than oneand a half million inhabitants of the Gaza Strip. The discharge of partially treated anduntreated waste water along the shores of Gaza strip is considered as the main sourceof pollution in this region. In the Gaza strip, there are five main treatment plants (BeitLahia, Gaza, Rafah, Khanyounis in addition to the most recently established plants atWadi Gaza) for collecting and treating wastewater to the level allowed to be disposedsafely and to not pollute the environment. The efficiencies of these treatment plantshowever are very low and their effluents are mostly discharged into the MediterraneanSea either untreated or partially treated. Often, the reason is financial. Additionally,the population growth without a proper expansion of the treatment plants has alsolimited the efficiency of these treatment plants. According to UN report, theMediterranean Sea receives daily between 50 and 60 million liters of partially treatedand untreated sewage from the Gaza strip. As a result, the beaches in front of Gazacity, Beach Camp, Azzahra City and Deir El-Balah are heavily polluted by sewagedischarges. Such pollution presents a major health risk for swimmers and marine life.Several studies have been conducted for evaluation of the environmental situations;these showed heavily contaminated recreational seawater along the seashore.