Sirsiya River is among the most polluted rivers in Nepal. It lies in the Birgunj-Pathlaiya corridor which flows from North to South crossing the border from Nepal to India.
The river flows pristine from upstream and then industrial effluents and sewage flows into this river. This river corridor has hundreds of industries among which textile, and leather tannery are among the most polluting of them.
An Easy-to-Understand PDF relating to the problem of the black-odorous Sirsiya River, its reasons, and the potential solutions.
The Reason for Sirsiya River to be pitch black
The first thing to know is what is causing the pollution. From many lab tests, many parameters exceed the standard value. The main reasons for this pristine river to be black are:
1. HIGH BOD: The BOD5 and COD at some river points were found to be as high as 404 mg/Liter and the BOD5/COD ratio was 0.76. This means that there is rich digestable organic matter.
The BOD5 decays downstream and gets reduced from 404 to 285 to 73.5 to finally at 43.8 mg/liter at Thakuram campus back Sirsiya river bridge. Here, although the pollution parameters were satisfactory at the lower points, the damage has been done and the river resulted in pitch-black river.
The tannery and textile dying industries are among the main polluters. However, it is not limited to these. There are hundreds of industries like textiles, pharmaceuticals, steels, leather goods, chemicals, liquors, ghee, and soaps that discharge their effluent in this river some with treatment and mostly without treating it.
The domestic sewage is also discharged into this river just with primary treatment in household septic tanks.
2. LOW DISSOLVED OXYGEN (DO): The river has virtually zero dissolved oxygen at many points. The DO is required for the natural biological treatment to be aerobic in nature which produces brownish settlable sludge.
The lack of dissolved oxygen in the river causes the biological process to be anaerobic. This is the same process that occurs in the septic tanks of our homes.
The anaerobic digestion of organic substances occurs with the help of anaerobic bacteria in the absence of dissolved oxygen. This also produces ammonia which is found to be as high as 8mg/liter which generally confirms the anaerobic process occurring in this river.
This anaerobic process produces the humic substance which acts as black pigments. Although the water is completely black, its turbidity is as low as 12.6 because this pigment is indicated by color and not by turbidity.
3. COLOR MIXING: Many textile dying and tanning industries discharge untreated sewage into the river. These industries not only discharge colored dyes in effluent but also produce higher quantities of higher discharge added with higher BOD.
This higher discharge means the effluent treatment plants need to be of bigger sizes and dyes need a more complex treatment method. Hence, these textile dying and leather tanning industries are extremely reluctant to make an effluent treatment plant due to the need for bigger treatment plants.
The black river can be explained by the color theory. Mixing of red, blue, and green produces a perfect pitch-black pigment.
The leather tanning uses chromium sulphate (GREEN) along with excessive lime and sodium chloride salt which gets released in its effluent. However, with the market conditions chrome tanning has been stopped for now, and only liming is done in these leather tanneries to peel off excess skin and fur, and is dried afterward. Even then high organic matter is released that a high biological oxygen demand.
The textile dying uses a lot of colored pigments and different chemicals which also have a high BOD (range: 242 mg/liter) and COD (range: 666 mg/liter).
This mixing of colors produces a perfect black pigment dissolved in water. Just like turmeric pigment curcumin gives a yellow color, this black pigment makes the river water perfectly black. The textile dyeing industry also contributes to humic acid with its high BOD thus making it black. This makes the textile industry the worst enemy of this river.
Another similar example of this kind of pollution is seen in the Buriganga river in Bangladesh which has a similar pollution pattern to that of the Sirsiya River.
Potential Solution to revive this dead river
The river is a dead one with no aquatic life in it. However, this river can be revived again with potential remediation actions. The solution lies within the problem.
1. REDUCE THE POLLUTION: The river has high organic pollution that needs to be controlled. The primary target should be high BOD requiring the industry’s effluent. It should either be treated within the industries and low BOD water should only be discharged into the river. The textile and tannery should be targeted in the first step.
Reducing pollution is the most important step that has the most impact. All rivers have a natural self-cleaning property up to a certain level. Exceeding that capacity like happening in this river now has been the main issue.
2. AERATE THE RIVER: To fulfill the insufficient dissolved oxygen (DO) in the river and to ensure the aerobic process happening, natural or artificial aeration methods can be adopted.
Aeration is simply mixing air and water which increases the dissolved oxygen in water. This improves the self-cleaning ability of the Sirsiya River.
The murky brown color of the water is also not fully clean, but due to its aerobic nature due to aeration, the septic condition is prevented and hence the color is not black.
The warm waters hold lesser dissolved oxygen as temperature becomes very high in summer in these areas. The flat Terai region has a gentle slope hence the Sirsiya River has very little turbulence to aerate itself.
The aerobic bacteria need dissolved oxygen to eat and digest the organic matter for the aerobic process. Increasing the DO naturally or mechanically will improve the river water quality.
3. ENGINEERED NATURAL TREATMENT SYSTEMS: There are engineered treatment systems that can be applied to simultaneously increase the dissolved oxygen, treat the organics, and uptake the nutrients.
There are complex methods like activated sludge process, trickling filter, MBBR, and others. However, these need high capital investment, high operating costs, and skilled manpower.
Hence, decentralized natural treatment methods like constructed wetlands and floating wetlands are the best-suited engineered methods to treat the water in the river. These attached growth methods will give better results and at a faster rate than the is treated naturally due to higher concentrations of microbes in biofilms.
These are cheap and easy ways to treat the polluted river. The constructed wetland in particular can be used to treat the effluent from village drains near the river banks as shown in the figure below. The floating wetland can also be directly kept in the Sirsiya River itself. However, it will be problematic during rainy season and diversion is better suited for this river.
4. CHECK FOR QUALITY AND MONITORING: Regular water quality monitoring is essential to ensure that good conditions are met. This also finds the pollution and the polluter can be penalized.
Again, this maintains the record of the state of the river and identifies the pollution as soon as it happens. Added with regular monitoring, the river quality can be protected.
Only one process cannot make the Sirsiya River back to its original form. All the discussed processes should work together to bring back the river to its original state. In reality, the river will not return to its drinking water state. However, it can be made much better and pleasant to the eye with prospering aquatic life in it.
The reason why the Sisiya River isn’t black during the Chhat festival
An interesting thing happens during the Chhat festival in this river. The river suddenly appears to be more clean in a matter of days. During this festival, most of the industries along this river corridor are closed.
The rice husk ash remains along the river banks and the domestic sewage continues to get dumped into this river. However, by stopping the industrial effluents from entering into this river, the river is not black. This justifies that the black color of the river is mostly due to industrial pollution.
A bit more technical reason is that when the river gets lesser organic loads (lesser BOD), the river has a better ability to uptake dissolved oxygen from the air and retain surplus dissolved oxygen in the water. This makes the digestion process to be aerobic in nature. Thus, the sewage appears to be murky brown rather than pitch black due to aerobic digestion.
In short, the river naturally tries to dissolve oxygen in water to its saturation state, but to high BOD, the DO becomes zero in the polluted situation.
As the BOD gets reduced which means reducing the pollution, the DO is in better condition to fulfill the BOD and to remain in surplus. This acts just like aeration used in wastewater treatment to treat the waste in a short time in a matter of a few hours.
If surplus dissolved oxygen can remain in the water, the aquatic life will return to this now-dead river. The appearance of fish in this river will be an indication of ultimate success.
A thumb rule is: “If there is fish, the river is not toxic and clean”.
Also Read: Sirsiya River’s Black Water: Finding Solutions
Rakesh Shah (Civil and Environmental Engineer)
Author