Durban Metro: Deep Sea Sewage Outfalls

Durban Metro : Deep Sea Sewage Outfalls Page 1

Presented at the WISA 2000 Conference, Sun City, South Africa, 28 May - 1 June 2000

DURBAN METRO : DEEP SEA SEWAGE OUTFALLS

Tony BaileyDurban Metro Water Services, Laboratory, PO Box 1038, Durban, 4000

Tel: (031) 302 4793 Fax: (031) 302 4747 E-mail: [email protected]

SUMMARY

The Durban Metropolitan Council operates two deep sea sewage outfalls. These outfalls were constructed in1968/69 and are located off the Bluff coast. Prior to the selection of deep sea discharge of sewage as a viabledisposal option a detailed environmental assessment was conducted, with particular emphasis on the selectionof the most suitable location for the outfalls.

The impact of the sea outfall discharge on the marine environment is of cardinal importance to the Council, inparticular their continued viability and success. Since 1962 to the present, the CSIR has conducted monitoringprogrammes at and adjacent to the sea outfall sites. These monitoring programmes are very intensive andinclusive and assess all aspects of the marine and coastal environment which may present evidence of stress tothe aquatic eco-system.

The management of these outfalls and other contributing land based activities plays a crucial role in ensuring theeffluents discharged do not compromise the environment. Control of industrial effluent and the application of theCouncil’s Sewage Disposal By-laws is such that potential harmful effluents are prevented from being discharged.Industrialists also contribute by introducing waste minimisation practises and better effluent management.

The coastline of Durban is one of the city’s major assets and it is in Durban’s best interests to ensure the coastalenvironment is not compromised in any way. This paper will demonstrate that correctly located, designed,constructed and managed deep sea outfalls can contribute to the overall success of wastewater managementwithin a coastal metropolitan area.

1. INTRODUCTION

In 1889, John Fletcher, the Durban Borough Engineer proposed a sewerage disposal system for Durban,whereby water borne sewage would be discharged to sea at Durban’s Point. On 1st July 1896, screenedsewage was discharged via an outfall in the main channel at the harbour entrance during the first fewhours of the ebb tide. This enabled the establishment of the water borne sewerage system that nowextends throughout the Durban Metro area. This outfall site was to serve Durban’s residents for manyyears.

In 1938, the Council approved 2 new projects to expand the capacity of Durban’s sewerage system.Unfortunately World War II intervened and no action on these proposals took place until 1957. By thistime the Point - Harbour discharge was unable to meet the needs of the ever increasing population ofthe City. Dilution levels at the harbour mouth were dropping rapidly as the volume of sewage dischargedduring the ebb tide increased.


Figure 1 : Map indicating the location of the 2 deep sea outfalls, samplingsites, sampling grids at each diffuser and adjacent beach sample sites. Source: CSIR 2

2. THE DEEP SEA OUTFALLS

The Council proposed the construction of 2 deep sea outfalls off the Bluff and after support fromoverseas consultants it was agreed to proceed with this proposal. The Council for Scientific andIndustrial Research (CSIR) was commissioned in 1962 to perform a detailed study of the proposed seaoutfall sites. The study was performed by the Oceanic Research Group of the National PhysicalResearch Laboratory and the National Institute of Water Research (NIWR). The final report waspublished in September 1965. The study concluded that the oceanographic conditions and coastline inthe vicinity of the proposed outfalls were very suitable for the construction of the outfalls. The lay aspectsare the presence of a strong litoral drift and a steep drop off of the sea floor enabling the constructionof a deep sea outfall within an economic distance from the shore.


One outfall entailed the construction of a tunnel under the harbour mouth from the old Point disposal siteto a new works on the northern point of the Bluff, the Central Works Outfall. The second outfall wouldbe constructed from a new works on the northern bank of the Umlaas Canal, the Southern Works Outfall.Construction of the land based infrastructure commenced in 1962. Considerable difficulties wereexperienced during construction of the outfalls due to severe storms, and several lives were lost duringconstruction. The Southern outfall was completed in November 1968 and the Central outfall in January1969.

Table 1 : Dimensions of the deep sea outfalls

CENTRAL WORKSOUTFALL Specification SOUTHERN WORKS

OUTFALL3.2 km Length 4.2 km1.23 m Main diameter 1.37 m422 m Length of diffuser section 422 m

18 Number of diffusers 3448 m to 53 m Average depth at diffuser 54 m to 64 m

135 000 m3/day Design capacity 230 000 m3/day

3. TREATMENT PROCESS

Raw sewage at both works was subjected to conventional screening, de-gritting, removal of floatablesand primary sedimentation. Settled sewage was then discharged to sea via the outfalls.

At Southern Works, the removed sludge was thickened, anaerobically digested and mechanicallydewatered, prior to being disposed of on site. At Central Works the removed sludge was thickened,dewatered and incinerated in an on-site oil fired incinerator. The ash was disposed of on site.

In 1979 the Durban Corporation submitted a proposal to the Water Research Commission (WRC) toinvestigate the viability of re-introducing the sewage sludge to the settled sewage and discharging thismixture through the outfalls. This proposal was based on the ever increasing cost of diesel and thedisposal to land. It was determined that this option would have many benefits to industry and ratepayersof Durban.

With approval from the Department of Environment Affairs and Fisheries, the Durban Corporation, WRCand NIWR embarked on a 2 year programme to study the impact of this disposal option on the marineenvironment and adjacent coastline. The investigation commenced in October 1980. Althoughnecessary discharge permits and authorisation had been obtained, the public outcry when the projectwas announced was so vociferous that work was stopped in February 1981. There was widespreadpublic concern about potential pollution of beaches and impact on marine life. Some Bluff residents wereso incensed that they threatened to blow up the pipelines rather than allow the project to continue. Manypublic meetings and forums were held to debate the issue and eventually after the Durban Councilapproved, by only 1 vote, the project recommenced in June 1981. For the first 12 months the mixture wasdischarged from the Central Outfall with the Southern Outfall acting as a control. For the final 12 monthsthe mixture was discharged from the Southern Outfall with the Central Outfall being the control.


Figure 2 : Settled sewage and sludge discharged to sea

The final report on the investigation, Detailed Report : Sludge Disposal to Sea (1983) 6 was released andconcluded that sewage sludge could be co-disposed with settled sewage through the deep sea outfallswith no detrimental impact on the marine environment and adjacent coastline.

Consequently the Durban Corporation applied for and was granted permits by the necessary nationalauthorities to continue disposing the settled sewage and sewage sludge mixture via the deep sea outfalls. These permits are still valid and have been modified and upgraded after each re-application. In termsof these permits the Durban Council must employ external agents (the CSIR) to conduct detailed annualsurveys of the marine environment and the coastline adjacent to the outfalls. An under water inspectionof the pipeline must be performed, by divers, every 2 years. A full video record is made of this inspection.The divers also take sediment samples at selected sites around the diffusers. The permit also requiresspecific land-based activities to monitor and control effluents which are destined for the sea outfalls.

4. ANNUAL SURVEYS

The CSIR (in its various guises) has been involved in monitoring the sea and coastline adjacent to theoutfalls since 1962. The current monitoring program, “Sea Disposal of Sewage : Environmental Surveysin the Durban Outfalls Region” , has been ongoing since 1983. This survey is required in terms of theoutfall permits and is concerned with : (i) Water quality (chemical, physical and bacteriological aspects),(ii) sediment quality (physical and chemical aspects), and (iii) the biota (benthic communities and themeasurement of any accumulation of chemicals by organisms through exposure in the environment).Sampling strategies involve a grid of sample sites around each sea outfall for the collection of water,sediment and biota samples, sampling of the water column at various depths above each of the outfalls,and samples of water, sediment and biota collected on the adjacent shoreline. Control sample sites tobe used for comparisons are included in the sampling strategy. Samples are also collected from thetreatment works. Refer to figure 1 for the location of these sample sites.

As technology has improved, the methodology for most of the analyses performed has been adaptedand enhanced. As new developments occur and additional environmental concerns are identified, thesurveys have been expanded and refined to ensure all potentially detrimental impacts the sea dischargesmay have on the environment are included in the study.


As technology has improved, the methodology for most of the analyses performed has been adaptedand enhanced. As new developments occur and additional environmental concerns are identified, thesurveys have been expanded and refined to ensure all potentially detrimental impacts the sea dischargesmay have on the environment are included in the study.

The annual survey includes the following sections. Extracts from the conclusion of each of the sectionsof the latest CSIR report (1998) are indicated in italics.

` MicrobiologySurvey of the water column in the vicinity of each diffuser, sediments in the vicinity of the outfalls,analysis of the works’ effluents, assessment of shellfish (mussels and oysters) collected from therocks in the littoral zone and surf waters at adjacent beaches. ~131 samples.“The 1998 microbiological survey confirmed that the Bluff beaches continue to exhibit surf waterof excellent quality and bivalves free of pathogenic organisms.”

` Viruses and ColiphagesSamples collected from sites near the outfalls and adjacent beach sample stations are examinedfor viruses and coliphages. ~12 samples.“The continued discharge of sewage via the submarine outfalls produced no measurable adversechange regarding viruses and coliphages in their vicinity.”

` Benthic MacrofaunaMacrobenthos, typically a wide range of invertebrates are sampled from the sea bed in a gridpattern around each outfall. The species examined tend to be sedentary and have relatively longlife spans. Effects of the outfall will be observed from changes recorded in the species diversityand numbers of these species at the sampling sites. ~60 samples.“There was no evidence in the 1998 benthic macrofauna surveys to suggest that the Central orSouthern Works Outfalls were exerting any significant ecological impact on the seabed. Bothoutfalls appear to be functioning efficiently.”

` Trace metalsWater samples are collected at various depths in the diffuser zone of the pipelines, sea bedsediment samples from a grid pattern at each diffuser section and samples of water, sedimentand biological specimens are taken from adjacent beaches for metal analysis. ~ 113 samples.“All metals in seawater were low.” “Trace metals in beach sediments were all generally low during this study period.”

` Chemical surveysWater and sediment samples from the vicinity of the outfalls and adjacent Bluff beaches aresampled for general chemical analysis. ~104 samples.“In general, it appears that the outfalls have no detrimental effect on the chemical conditions inthe survey area during the period under review.”

` Chlorinated hydrocarbonsSamples of the effluent discharged to sea, sediments around the outfalls and mussels andoysters from the adjacent beaches are collected for analysis. ~46 samples.“Chlorinated hydrocarbons and PCB values obtained in the 1998 survey indicate no adverseinfluence directly attributable to the Central or Southern Works discharges into the sea.”


Figure 3 : Durban’s beach sampling sites. Source: CSIR 3

` Effluent toxicitySamples are collected from the settling tanks at each of the treatment works for toxicity testingusing sea urchins. ~6 samples.“During the year 1998, two samples from the Central Works, and four from Southern Works wereall within the normal toxicity expected from the sea urchin test with sewage effluent.”

5. RESULTS OF SURVEYS

It is not possible to include in this paper all the data obtained from all the surveys performed on Durban’sdeep sea outfalls. What all the annual surveys have in common is that no data has been obtained thathas shown the outfall discharges are the source of any detrimental impact on the marine environmentor adjacent coastline. The conclusions extracted from the 1998 annual survey included above areindicative of the conclusions and comments from all reports over the years.

This does not imply that there has not been results or data obtained which identify problems with themarine environment or adjacent beaches. On the contrary, these CSIR sea outfall surveys, plus otherexternally commissioned surveys and the Metro’s own monitoring programs have produced some veryinteresting results over the years. This data has been invaluable for the overall assessment of theenvironment in the vicinity of the sea outfalls. In all cases the source of any problems has never beenidentified to be the deep sea outfalls.

` Beaches

The main concern expressed by residentsof Durban, both in the past and at present,towards the sea outfalls has been thepotential they may have to create pollutionon the City’s beaches. An isogram (Figure4), extracted from a CSIR report producedon behalf of the Durban Metro whichmonitors the quality of water on all beacheson the Durban coast, contains a great dealof information which supports the findingsthat the sea outfalls have little or no impacton the Durban beaches.

The data in the isogram is for beachsample stations as displayed in Figure 3.Stations numbered 1 to 8 are along the“Golden Mile” to the north of the harbourentrance (Sample X). Stations 9 to 26 arealong the Bluff coastline adjacent to the seaoutfalls. The beach water qualityclassification used in the isogram isexpressed as Class I (excellent), to ClassIV (poor). The classification system ispredominantly based on the microbiologicalquality of the samples analysed.


Figure 4 : Isogram depicting the quality classification of Durban’sbeaches. The horizontal axis showing the sample stations - refer Figure3, and the vertical axis the years from 1964 to 1998. Source: CSIR 3

The sample collected at the harbour mouth, Station X, from 1964 to 1969 was always classified as ClassIV. This poor quality water is due to the location of the old sea outfall. The impact of the old sea outfallon the beach water quality at Durban’s premier bathing beaches to the north of the harbour entrance canbe clearly observed. The quality classification of the beach stations numbered 1 to 8 prior to 1970 wasconsistently poor. After the closure of the old sea outfall the surf water quality of Durban’s northernbeaches improved significantly. During the 60's and 70's there were many beach pipes (some illegal)and storm water drains which flowed across the beaches into the surf. Improved management of thestorm water and closure/diversion, where possible, of the beach pipes has significantly reduced theseas a source of beach contamination.

The isogram also indicates consistent poor beach water quality at several stations. All these are locatedadjacent to river mouths, the Mgeni River (1, 1a, 1b), Mlaas Canal (19, 20, 21), Reunion Canal (22, 23)and Sipingo River (25). This highlights the major source of contaminated beach surf water, namelyterrestrial water. The stations located furthest from sources of terrestrial water invariably had surf waterof the highest quality. This contamination is predominantly due to housing, infrastructure and sanitationproblems experienced in the river catchments. The lack of sanitation services, abuse of these servicesand overloading of the sewerage systems contributes to very high levels of sewage in the local rivers.The Council is involved in development and remedial programs that are systematically reducing theseproblems and the contamination of beaches is becoming less.


Figure 5 : Graph of mercury levels in sea outfall effluents

The generally poor water quality in 1987 is as a consequence of the severe flooding and heavy rainfallsin KwaZulu Natal. Poor surf water quality was experienced for many months after the floods as riversflowed strongly into the sea.

`̀̀̀ Biota

The measurement of the numbers and variety of biota sampled in the vicinity of the outfalls has shownno significant variations over the past 20 years. The Natal coastline is notoriously nutrient deficient, butthe nutrients in the sewage discharged has resulted in a more dynamic and diverse marine populationin the vicinity of the outfalls. Upwards of 100 different taxa (species) have been identified on the seabedaround the diffusers. It is debatable if this diversity of species would be present if the outfalls were notthere.

` Sediments

Although elevated levels of silt (derived from the co-discharged sludge) have been identified onoccasions within the sampling grids, the amount of silt has not reached levels that would create anyconcern. Significant increases in silt levels have been recorded on the north side of the Central Outfallsampling grid (site c3 in Figure 1) over the past few years. This has been accompanied by an increasein nutrients. Investigation showed that this sample site is immediately adjacent to the Portnet dump sitewhere they discharge silt dredged from Durban harbour. The silt levels have generally returned to normalwhen the dredger dumping site was changed, indicating the silt is not due to the sea outfall. ` Metals

On several occasions it has been observedthat certain metals have increased inconcentration. Generally more attention ispaid to this metal or metals during the nextyear’s survey to establish if any potentialthreat to the environment exists. In1990/91 concern about elevated mercurylevels in the sea outfall effluents wasexpressed. Although the levels of mercuryin the effluent were well within therequirements of the discharge permits,they were higher than in previous surveys.The concern lead to the purchase of betteranalytical instrumentation to improve theanalysis of mercury in the effluents. More frequent monitoring of effluentsentering the works was performed. Although no reason for these higher levels,or the latter reduction in levels was ever identified, the graph in figure 5 shows that the effluent mercurylevels were less than 0.2ug/l in 1996.

In the mid 1990's higher than normal levels of chrome were found in the samples from the beachadjacent to the Reunion Canal. Extensive investigation indicated an old land fill site in the catchment ofthis canal was leaching chrome after periods of heavy rainfall.


0

100

200

300

400

500

ug/l

Hg *100 Cd *10 Cr Ni Cu Pb Zn

1994 1995 1996 1997 1998

Southern Works Sea DischargeAnnual mean metal concentrations

Figure 6 : Metal concentrations in Central and Southern Works SeaDischarge effluents.

0

100

200

300

400

500

ug/l

Hg *100 Cd *10 Cr Ni Cu Pb Zn

1994 1995 1996 1997 1998

Central Works Sea DischargeAnnual mean metal concentrations

All metals, especially those known to accumulate in the environment are routinely analysed in all the seaoutfall effluents. The results obtained are all well within internationally acceptable levels.

6. LAND BASED ACTIVITIES

The establishment of the Durban Metro over the past several years has consolidated the activities of thePollution Control Division of Wastewater Management leading to a more comprehensive application andmanagement of the new Durban Metro Sewage By-laws throughout the metropolitan area. This hasensured all effluents discharged to Metro Wastewater Treatment Works comply in all respects to theeffluent limits listed in the By-laws.

The Pollution Inspectorate has developed a good rapport with local industry and work together to improvethe effluent quality discharged to our treatment works. This has been successful, as can be seen in theongoing improvement (reduction) in the metals measured in the works effluents that are discharged tosea. Refer Figure 6. This reduction has not only been limited to the metals in effluents, but encompassesall undesirable contaminants.

No permits are issued for thedischarge of any effluent to a seaoutfall unless a full audit of theindustry and its effluent/s has beenconducted. In certain cases theeffluent must undergo toxicity testsconducted by the CSIR using seaurchins. This generally ensures thatno potentially deleterious effluent isknowingly allowed in the finaldischarge to sea.

Local industrialists are also realisingthe cost benefits of wasteminimalisation and effective wastemanagement. The emergence ofISO 14000 as a desirablecertification has also contributed tobetter waste management andcontrol. This all contributes to“better” effluent for disposal via thesea outfalls.

The Design Branch, WastewaterManagement has the responsibilityof managing and scheduling theunder water inspections andthereafter arranging for any repairsor modifications that may berequired.


The Treatment Works Branch perform all day-to-day activities at each of the treatment works dischargingthrough the sea outfalls. Their tasks involve routine maintenance and flow management to ensure amore consistent effluent is discharged via the sea outfalls. The Operations staff perform all cathodicprotection tests to ensure the sea outfalls are not damaged by the highly corrosive sea water.

7. CONCLUSION

Durban Metro Water Services, Wastewater Management recognises the potential for environmentalproblems that could arise from sea outfalls and the experiences of the problems that occurred due to theold harbour mouth discharge will not easily be forgotten. Any sea outfall must be ideally located andeffectively managed to ensure a minimal impact on the marine and coastal environment. Durban isfortunate to have two such sea outfalls. Extensive, long term environmental monitoring has proven thatDurban’s deep sea outfalls in no way impact negatively or significantly on the marine environment oradjacent coastline. The sea outfalls are subject to strict permit requirements and are intensivelymonitored and well managed.

Durban’s coastline is one of the City’s major tourist assets and it is Durban’s best interests that the deepsea outfalls not be allowed to negatively impact on this asset.

8. ACKNOWLEDGMENTS

The author would like to thank all the Staff in the Laboratory and Wastewater Management who contributed to thework presented in this paper. Special thanks to the staff of the CSIR for their valuable information. FrankStevens, Director, Wastewater Management Department is thanked for granting permission to present this paper.

9. REFERENCES

1. They Built a City. Durban City Engineers Department 1882 - 1982. Rory Lynsky

2. Sea Disposal of Sewage : Environmental surveys in the Durban Outfalls Region. Report Numbers 2 to17 (1984 to 1998). CSIR Environmentek, East Coast programme (et al)

3. Surveys Monitoring the Sea and Beaches in the Vicinity of Durban. Report No. 28. Surveys made in 1998.CSIR - KwaZulu Natal.

4. Sea Disposal of Sewage : Environmental surveys in the Durban Outfalls Region. Minutes from variousmeetings 1995 to 1999.

5. Detailed Report : Sludge Disposal to Sea (plus appendices). National Institute for Water Researchin collaboration with the National Research Institute for Oceanology the Durban Corporation and the WaterResearch commission. September 1983.

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Durban Metro: Deep Sea Sewage Outfalls