Tidal Flushing Characteristics of Municipal and Industrial Waste in the Karachi Coastal Waters and Simulation of Waste Field Dilution at Sewage Outfall.
The deterioration of Karachi coast and the marine environment near to it cannot be ignored. Effluents from industries and untreated sewage discharge into the coastal waters through Malir and LyariRivers and this ultimately have a negative impact on marine life and also to human health. The toxicants present in the industrial wastewater play a vital role in declining the marine population and also cause contamination.
The knowledge of flushing and residence times of estuaries and creeks is very important. However, the estimated flushing time by using the freshwater fraction method ranges 15-20 days at both sites. The freshwater fraction method is considered to be a reasonable estimation method as in this method salinity variation in the estuary and open sea are also incorporated in the calculation. Discharging the effluents in the estuary results the destruction of the ecosystem of the area. Thus the discharge of polluted water directly in the deep offshore water certainly improves the quality of life in the coastal areas. Therefore, submarine outfall construction is an effective way to discharge the sewage far away from the coast where diffusion, dispersion and dilution are enhanced.
This paper deals with the estimation of flushing time scales and marine environmental parameters in the offshore waters of HawkesBay and Gizri creek/DHA coastlines. The outfall model, DESCAR, is used to determine the spreading and dilution of pollutants under different oceanographic and meteorological forcing. The other outfall parameters such as Initial dilution and Froude numbers and return back travel time are also estimated by using USEPA spread sheet and the ocean-atmosphere archived data.
Keywords: Lyari and MalirRiver, Residence time, Flushing time, Submarine outfall, Initial dilution, Outfall model.
Now a day's industrial and domestic discharge are playing a vital role in declining marine life worldwide . Karachi being an industrial hub and populated city of the country does not have the facility for treating waste water before it is dumped into the sea . Approximately 450 MGD of raw municipal sewage and industrial effluents are being dumped into Karachi harbor estuary and the Gizri creek area through Lyari and Malir rivers respectively . Karachi Harbor and Manora channel constitute the most heavily polluted marine sites in Pakistan , in which major pollutants are heavy metals . The contamination in the coastal waters plays important role in declining the marine populations, causes contamination and consequently resulting in economic losses .
The untreated municipal and Industrial wastewater containing a variety of marine pollutants which ultimately have a negative impact on the ecosystem of an area and poses serious health hazards . The flushing and residence times of estuaries and creeks are important parameters in determining the exposure period of pollutants to marine organisms and its consequent impact on the marine ecosystem. Estuaries with shorter flushing times are better able to accept effluents because mixing of fresh and salt-water is greater and dilution of polluting substances is better . Similarly, greater tidal volume enhances the dilution and dispersion processes in the coastal environment. About 50 billion cubic meters of sea water enters and leaves the estuary during a tidal cycle.
The wave action and ocean currents disperse pollutants all over the coast and even the high tide condition may favor the pollutants to return back to the shore. Therefore, release of pollutants in the deep offshore area through submarine outfall is an effective and sustainable way to improve the quality of life in the coastal areas which helps in the enhancement of dispersion, dilution of sewage water that have minimal environmental impacts . Thus submarine outfalls are the best option for coastal megacities to release sewage and industrial waste in the ocean.
Pollution parameters such as BOD, COD, TSS, Phenol and heavy metals play an important role in determining the pollution profile of a certain area. The DESCAR outfall model is used to simulate the pollution parameters at off Hawkes bay site for Lyari river discharge. The model is run for both high tide and low tide conditions and dispersion and dilution of pollutants under oceanographic forcing are observed.
Karachi is producing more than 450 million gallons of wastewater per day and this wastewater is carried by the two main rivers. The Lyari river passes through the center of the city and Malir river covers the eastern part of the city. These rivers discharge sewage waters into the semi enclosed coastal water bodies. Thus the coastal zone of Karachi receives entire sewage and industrial waste of the city and this is leading to a contaminated coastal zone and increased public health risk in Karachi and adjoining coastal areas.
In this study, main area of interest is Karachi coast and adjoining areas. The study area is shown in Fig. 1. The bathymetry of the study area indicates a relatively shallow area at the mouth of the dominant tributaries, Gizri and Phitti creeks and Lyari river outfall . However, scouring action by enhanced tidal currents continues to deepen the channel and at some points it is more than 20 m deep. The transport of pollutants towards the open sea during low tides is shown by arrows.
Oceanographic and Meteorological Parameters of the study area
The climate of Karachi and surrounding coastal waters is subtropical and humid, hot and dry condition prevails with seasonal wind variations. The mean monthly air temperatures vary from 18 oC to 32 oC in winter and summer respectively. In general, temperature variations show four normal phases, warming from February to May, cooling from May to August, warming from September to November and cooling from December to January. The mean monthly temperature in Karachi has the highest amplitude of variations with maximum in May and minimum in January. The study area lies under the influence of the Indian monsoon system. The mild winter in the NE monsoon and relatively strong SW monsoon system prevail during winter and summer seasons respectively. Regular reversal of wind pattern under the influence of two monsoon systems affects Karachi and the surrounding region. During the SW monsoon season the strong southwesterly wind prevails and reaches up to 25 Knots.
The maximum precipitation falls in the summer monsoon season with an average rain of about 256 mm/year. During winter the weaker north easterly winds blow. The transition period between the two monsoon seasons shows variable wind speed and direction. The high humidity of about 85% is observed in the summer monsoon season .
The southwest monsoon winds having an average speed of about 15 m/sec generate strong waves and current in the area. The strong and persistent southwest monsoon winds generate swell waves with a period of about 12 seconds, which strike Karachi coastline from southwest direction. The current generated by winds or tides are of extreme importance for pollutant movement, dispersion and dilution of Industrial and domestic pollution in the coastal zone. The wave action and strong currents cause speedy flushing of pollutant and increase the dispersion and the dilution rate. However the movement of polluted sediments likely to spread in the areas which are supposed to be ecologically safe. During winter, northeast monsoon prevails in the region. During this season, predominantly northeasterly wind with an average speed of 5 m/s prevails. The wind from southwesterly direction is about 10% of the winds prevailing in the northeast monsoon season.
During the southwest monsoon, the prevailing current pattern is clockwise in the Arabian Sea. In the northeast monsoon season anticlockwise flow prevails. Predominantly currents with a speed of 0.2 0.5 m/s persist in the SW monsoon season, whereas in the northeast monsoon predominantly weak currents with the speed below 0.3 m/s prevail.
The current data of the study area were taken from the NIO and KPT [11, 12]. The strong currents range from 1.0 m/s to 0.88 m/s observed during flooding and ebbing at the mouth of harbor and creeks. The cross current at the mouth of the harbor is 0.9 m/s. The currents in the eastern backwater (Chinna creek) are quite low whereas in the western backwater, the current speed of about 0.6 -0.7 m/s have been recorded.
The current measurements in the offshore waters of Hawkes bay and Gizri creek/DHA are used in the outfall model to simulate the sewage disposal at the sites of marine outfall. The current pattern at the offshore waters of Hawkes Bay is shown in Fig. 2. The observed direction of currents at both sites matched with the common reversal of tides during the whole tidal cycles. In general tidal currents are stronger during mid-tide (middle of high tide and low tide conditions) and attain a maximum speed of about 0.30 m/Sec.
Tides along the Pakistan coast are semi- diurnal with diurnal inequality. The tidal range varies from 1.8 m to about 3.2 m, which indicates that the tidal amplitudes are greater in the Indus deltaic region and gradually decreasing towards west. The average tidal range at Karachi port is 2.3 m . The tidal currents during the ebbing attain higher speed than flooding time. This high tidal water in association with strong current, thus contributes to the erosion problem in deltaic region. The variations of tides at Karachi, Phitti creek mouth and at Pipri are presented in Fig. 3.
The ocean-atmosphere parameters are used to compute different environmental parameters and as outfall model input data. The ocean surface current, depth current profile, tidal elevations, the wind pattern, river discharges and other information about Karachi harbor and surrounding coastal waters are obtained from different technical reports and data archives of different organizations such as NIO, PMD and KPT, PQA. The available ocean data are both quantitative and qualitative pertaining to flood and ebb during spring and neap tides [11, 12, 14, 15]. The numerical model DESCAR is used to determine the dispersion and dilution of pollutants in the open The DESCAR software uses two different mathematical models: Buoyant jet model and Stratified model. The Buoyant model is ideal for pollutant discharges located in the proximities of the coast and in the rivers, (using little depth).
This model is based on a time-independent Gaussian equation which simulates the pollutant dispersion in the water. However, the stratified model takes into account the formation of the pycnocline in the sea. This model is ideal for outfall discharges in the sea (using a high depth value). The intense heating caused by sunlight results in the formation of a less dense, warmer layer of water floating on top of a much larger mass of cold, dense water. Where these two bodies of dense water meet there is a zone of rapid change in the water temperature, called the thermocline or pycnocline. Such a thermocline is a permanent feature of the offshore waters of Karachi. The stratification phenomenon is the existence of two homogeneous water layers and separated by a thin thermocline layer. In this stratified water there is no exchange of pollutants through this pycnocline layer.
Since, Karachi offshore water is stratified, the stratified model is used here to simulate the spatial variations of concentration in the proximities of outfall. The program calculates the pollutant concentration in each point of the water considering each one of the pollutant sources and the conditions of the water. The details of structure of numerical models and mathematical algorithms are given in the .
Results and Discussion
Vertical Stability of Coastal waters of Karachi
A commonly used index of the static stability of the water column, suggested by Hesselberg and Sverdrup  is used to determine the stability of the offshore waters of Karachi. The estimated stability of the offshore waters of Hawkes Bay and DHA coast shown in Table-1 depicts a stable water column. This equilibrium stability of offshore waters shows the thorough mixing of water and no halocline and thermocline observed in the near shore waters of Karachi . The stability of the offshore waters of Karachi shows that both sites are suitable for marine outfalls and these outfalls are the best option to protect our coastal waters from degradation and also in the conservation of marine related ecosystems.
Table-1: Vertical stability of offshore waters of Karachi coast.
Offshore waters of###(surface)###(surface)###(surface)
Offshore waters of###(surface)###(surface)###(surface)
Time Scales of Pollutants Transport
The flushing or residence times have been used in assessing water quality in an estuary, harbors and semi enclosed water bodies. The flushing and residence times are used to gain understandings of how quickly, on an average estuaries flush or retain the materials. The pollutants impact on the estuary fauna and flora depends on biochemical time scales. If they are longer than the flushing or residence, the less would be the nutrient absorbance by marine biota. The algal growth can be correlated with the longer residence time. Therefore, knowledge of transport scales of an estuary is important and beneficial for coastal managers .
There are several methods to calculate the flushing time [20, 21]. The tidal prism method is simple and easy to apply. It has been found that this method underestimates the flushing time because the assumption that a water body is well mixed is not always valid. The freshwater fraction method uses the spatial variability of salinity from estuary to open sea. This method gives reasonable results with the assumption that the estuary is non-stratified. Hence, this method suits to shallow water estuaries. The Residence time is the modification of the tidal prism method. The residence time is the time, required for the total conservative originally within the estuary to be reduced by a factor of 1/e (i.e. 0.37).
Therefore, flushing times of sewage waters of Karachi harbor and Gizri creek are calculated by modified tidal prism method [22, 23] and freshwater fraction method [20, 24]. Similarly, the residence time of both semi enclosed water bodies is calculated . The estimated flushing and residence times of Karachi harbor estuary and Gizri/Korangi creeks are given in Table-2. The mean tidal prism volume of the Karachi harbor, including backwaters and Gizri/Korangi creeks are 3.2 x 108/m3 and 1.3 x 108/m3 respectively. With the average low tide volumes of Karachi harbor and Gizri creek, the flushing time of both sites is about 2.5-3.0 tidal cycles. However, this method is based on the estuary volume due to tides. Since, fresh water discharge is not being used in the calculation; this is only true under the low flow or zero flow condition.
The flushing times obtained by this method are much longer than the tidal prism method. The estimated flushing times at both sites with this method depict that the load removed from the Karachi harbor estuary and Gizri/Korangi creek takes 30 tidal cycles to 46 tidal cycles respectively. This method gives reasonable results with the assumption that the estuary is non-stratified. But in the stratified estuary vertically average salinities used in the calculation may not be representative. Since, Karachi harbor and Korangi and Gizri creeks are comparatively shallow semi enclosed water bodies; this method may give reasonable results.
The residence time return flow factor (b) plays an important role to determine the duration of residence time. The factor (1-b) is equal to the tidal prism volume leaving the estuary during the ebb tide that is replaced with coastal waters prior to re- entering the system. The calculated residence time ranges 8-9.5 tidal for both Karachi harbor and Korangi and Gizri creek water bodies.
High flushing time indicates algal blooms; this is due to the fact that the faster the flushing time, the less would be the nutrient observance by marine flora. The flushing time calculated by the fresh water fraction method is more significant when it is to be calculated for an estuary.
Dispersion and Dilution of Marine Pollutants through Submarine Outfall Hawkes Bay area The addition of sewage into the near shore waters causes perturbation in the coastal ecosystems. The wastewater of Karachi is being flushed into the estuary and creeks directly through sewage rivers. The wastes discharge close to shore or in estuaries is causing problems to the coastal ecosystem. The better alternative for coastal cities is to discharge wastewater into deep ocean far away from the coast through submarine outfall .
When the wastewater releases from the submerged pipe it undergoes basic processes of mixing with ambient sea water. The diluted waste is carried with the ocean water either to the surface or bottom of the sea depending on the density stratification of the ocean. The mixing and spreading of effluent horizontally establishes sewage field. The current speed and natural turbulence of the sea help in the large scale mixing and spreading of the wastewater.
An attempt is also made in this paper to simulate the sewage outfall spread at off Hawkes Bay/Manora area [18, 9]. The off Hawkes Bay outfall site is one of the potential marine outfall sites for Lyari river sewage discharge. The DESCAR outfall model is used to simulate the sewage at outfall sites. The basic data such as proposed outfall characteristics, port diameters as well as a number of diffuser ports are taken from the study report . All the input data and parameters needed to run the DESCAR outfall model is given in Table-3.
The other variables, those which affect wastewater spread such as minimum and maximum dilutions under different tidal conditions and bathymetry of the sea are also calculated. The USEPA spreadsheet  is used to calculate the initial dilution during spring and neap tides for both sites. The estimated Froude number and average moving water dilution at proposed site are given in Table-4.
The large Froude Number depicts no sea water intrusion into the outfall pipe as higher exit velocity of sewage from the outfall into the sea bed prevents outfall pipe chocking. The calculated moving water dilution in the offshore area of Hawkes Bay is 175. The estimated high dilution values depict that the offshore area of Karachi is suited for wastewater disposal with a high capacity of dilution and mixing.
The observed pollution parameters, taken from study report are used as input data in the DESCAR outfall model . The model is run for parameters having a high concentration exceeding NEQS and believes to be hazardous to human health (Table-4). The model was run for both flood and ebb tide conditions. During flooding the spreading of sewage is toward the coast, whereas during ebb tide the pollutant transport towards south or southwest direction. The general direction and spreading of pollutant from the outfall site under the influence of tidal currents during high and low tides are are shown in Fig-4 (a-b). The DESCAR model output during high and low tidal conditions are given in Table-5.
Table-2: Flushing time of Karachi harbor estuary and Gizri/Korangi creeks, sewage outfalls of Lyari and Malir rivers.
###Low tide volume###Tidal prism###Flushing time in tidal cycle###Flushing time in tidal cycle###Residence Time
###x 108 (m3)###x 108 (m3)###(Modified tidal prism method)###(Freshwater fraction method)###in tidal cycle
Karachi harbor and
Table-3: Input data and parameters needed to run the DESCAR outfall model (NIO, 1993).
###Length of outfall Peak###Average Depth at###Average Ocean current###Diffuser pipe###Number of ports, and
###from coast###flow###outfall site###speed at outfall site###diameter###port diameter
###7 km from shoreline
Offshore Hawkes###84 ports and 250 mm
###6 km across harbor 13 m3/s###21 m###0.3 m/s###3m
Table-4: A typical relationship between the discharge, Froude number and moving water dilution at offshore of Hawkes Bay/Manora
###Location###Discharge (m3/s)###Depth (m) spring###Froude number###Moving water dilution
###Offshore Hawkes Bay /Manora###12.928###23.0###12.6745###175
Table-5: The pollutants parameters observed at the mouth of Lyari river and model simulation results, showing the diluted values at outfall site.
###Diluted Value (mg/L)###Diluted Value (mg/L)
###Parameters###Observed (mg/L)###NEQS (mg/L)
###High tide condition###Low tide condition
The diluted values of waste depict that waste would mix throughout depth more quickly. Thus the disposal area offers more favorable conditions for diluting liquid wastes so that the negative effects on the ecosystem are dubious. Moreover, the semidiurnal tides in Karachi help spreading of outfall effluent away from the coast with strong dispersion and dilution process including rapid FC-coliform die off rate [9, 29]. The physical structure and orientation of diffuser pipe and ports, plays important roles in the dilution and mixing of sewage load. Hence direct observations will considerably improve the model results.
Since, Karachi harbor and Korangi and Gizri creeks are comparatively shallow semi-enclosed water bodies, the estimated flushing time depicts that with the faster flushing time, the less would be the nutrient observance by marine flora. Consequently there will be no algal growth because of short residence time.
The ocean is stirred and mixed by turbulent processes associated with Winds, Waves, Currents and Buoyancy (density differences). The vertical stability of an oceanic water column depends on the vertical distribution of density. A water column is stably stratified if the density increases with depth. The estimated stability shows that the offshore water of Karachi is suitable for marine outfalls.
The DESCAR outfall model is used to simulate the sewage at outfall sites. The off Hawkes Bay outfall site is one of the potential marine outfall sites for Lyari river sewage discharge. The simulation results indicate that the sewage outfall spread follows the tidal currents. The sewage spreads toward coasts during high tides. However, most of the time semidiurnal tidal reversal currents favor the spreading of sewage loads towards open sea with high dilution dispersion rates. Furthermore, the estimated high dilution values depict that the offshore area of Karachi is suited for wastewater disposal with a high capacity of dilution and mixing. Hence, the construction of marine outfall is the best option to protect coastal waters from degradation and also in the conservation of marine related ecosystems including mangroves.
In Karachi, approximately 450 MGD of waste water is produced but, none of all three waste water treatment plants is in operation, hence all the waste is dumped directly into the sea through estuaries. The present situation of estuaries is alarming, the high flushing and residence times of industrial and municipal waste waters indicate algal blooms which eventually disturbs our flora and fauna. The offshore waters of Hawks Bay and D.H.A. coast depict stable water column. However, the natural turbulence of ocean waters helps in dispersion and dilution of waste. The semidiurnal tidal behavior favors spreading of outfall effluent towards open sea with rapid FC-coliform decay rate. Thus marine outfall can be an efficient and economical way of dumping waste waters in to deep ocean, so that estuaries around Karachi may probably be conserved.
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|Author:||Khan, Tariq Masood Ali; Abid, Kanza; Jaffery, Shumaila; Ali, Irfan; Zakai, Hafiz Muhammad Ali|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Apr 30, 2015|
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