A brief on the science of wetlands for wastewater

-by Shruti Syal

 

Having sampled the water from 3 nullahs over a span of a few months, and visited 10 sites, there are some trends can can be noticed in order to plan a full-fledged comparison at multiple sites, and gauge how the wetland could be placed at these.

 

WHAT’S THE WATER CONDITION?

The most obvious of them is that phosphorus, ammonia-nitrogen, chemical oxygen demand, and coliform counts are clearly in violation of the prescribed national or international standards for treated wastewater, especially that which is discharged into water bodies that are expected to nurture aquatic life.

 

At Chirag Delhi, water quality with respect to these parameters worsened as they passed the settlement, but that wasn’t always the case. The fouling was not limited to liquid waste, but copious amounts of coarse material like rubber tyres, plastic bags, PET bottles, glass, and rags. Sites like Taj Enclave and AGCR Enclave were not considered viable for the pilot because the solid waste management required would be extensive.

 

Since the nullahs carry everything from stormwater to untreated wastewater, unless the various inputs along the entire length of each drain is tracked, it is difficult to ascertain the constituent water characteristics, which complicates the level of dilutions required, particularly for the Biochemical Oxygen Demand. Given the failure of dilutions at the higher end of the spectrum of measuring BOD of polluted rivers, it is clear that BOD violations are there, although unquantified. Another parameter where the dilution issue cropped up were the coliform counts that were performed on a 100X diluted sample, after which photographs were taken and a grid superimposed on the pictures of the colonies, despite which they were immeasurable.

 

Judging by the COD values obtained being more than double that of the prescribed standard, the organic matter content varying between 25 and 40% which is actually the range delineated by CPHEEO for solid waste in India, and the gross violation of the standards for ammonia nitrogen and phosphorus, it can be concluded that there is definite need for intervention to improve the water quality.

 

WHAT WETLANDS ARE KNOWN TO DO

Constructed wetlands are known to increase microbial degradation and sedimentation for BOD and TSS reduction, enhance ammonification, nitrification, and denitrification for ammonia removal, absorb phosphorus, and filter/sediment pathogens. But it is a low-maintenance system, not a no-maintenance system, and the aim of involving the resident communities of these informal settlements in the process of wetland maintenance is with the idea of generating a sense of environmental stewardship that fosters reduced polluting activities and not just the execution of a few simple tasks to ensure that the system is functioning. The stretch of the Yamuna along Delhi and Agra may be labelled as ‘dead’, but as is the case with any ecological system, facilitating the natural ecology can resuscitate the system over the long (or very long) term.

 

WHAT’S BEEN HAPPENING IN NEPAL?

As outlined in the Nepalese case studies profiled in the UN-HABITAT manual for constructed wetlands, monitoring of the water quality at the inlet and outlet has shown an 80-100% removal efficiency of BOD, COD, TSS, and ammonia-nitrogen for all types of wastewater over a span of 6-10 years. An obvious concern on that figure is that it is not known what the inlet and outlet levels actually are.

 

Also, there are other differences in the conditions. For one, setting up low-cost wetlands in the open drains of Delhi would require the use of surface flow wetlands rather than the subsurface flow wetlands used in the Nepalese cases. This is because if a subsurface flow wetland is constructed, water flow regulation through the wetland would be a big challenge. The water would either have to be redirected to the sides at multiple points in order to facilitate the cleaning of the entire body of flowing water, or the water flow would have to be slowed down in its entirety. With a surface flow wetland also the flow must be regulated, but the extent of regulation will be starkly lower, therefore making the system manageable.

 

THE BASIC CONSTRUCTED WETLAND

Designs could be varied. But the basic layout is locating a relatively less undulating flat surface with a depth below 1 metre, placing an impervious cover to maintain the water at that location while the plants absorb the various elements that need to be drawn out, and deciding on a planting scheme. Based on the current assessment from the Chirag Delhi and IP Estate sites, the plants decided upon for surface wetlands need to remove phosphorus and ammonia, which would also reduce COD, and further tests would have to be done to check if there are specific metals that would have to be removed. Often, the likely metals that contaminate waters in Delhi are arsenic, iron, mercury, and in rare instances, selenium. With phosphorus, monitoring is particularly critical because phosphorus accumulation is more common, and without regular removal of the soil sediments where it accumulates, phosphorus could be transported downstream in the channel. Luckily, surface wetlands are particularly effective in ammonia volatilization, and pathogen removal. Before the actual construction, tests will have to be done to detect metal concentrations of common pollutants like iron, arsenic, and mercury.

 

Most documented cases of wetlands in South Asia are subsurface ones, and they’re largely planted with Phragmites. The planting scheme requires inputs from landscape architects, and depends on the elements we’re focused on removing.

 

In order for the wetland itself to work, the preliminary treatment, a screen and grit chamber is required at the inlet. This is followed by a septic tank to remove more solids, with a minimum retention time of at least 12 hours, and depending on TSS removal rate, the desludging interval could range from 3 months to a year. Again, there are several considerations to be evaluated before constructing the tank, and inputs will be sought from engineers.

 

This raises the omnipresent concern that is changing the use dynamics of the wastewater in the informal settlements. Right now inhabitants are content unloading loudly gurgling batches of wastewater and plastic through pipelines coming out of nowhere in a maze of houses, right into the open drain, falling over mounds of already discarded trash. In a city wrought with solid waste management woes, it will have to be ACWUS’ imperative to arrange for an alternative for their trash collection or the idea will collapse.

 

WHERE THE EXPERTS COME IN

For the construction, inputs from environmental engineers and landscape architects will be needed. The considerations on the basis of which their inputs must be evaluated are outlined briefly:

1. The septic tank should be constructed based on the average wastewater volume, required hydraulic retention time, and with careful observation of the actual sludge and scum accumulation rate.

2. This is then used to decide wetland size, along with the 5-day BOD removal, and population size. The latter is the toughest to discern in the case of these nallahs because the drains have to be sourced back to their origins, and all the catalogued and un-catalogued inputs into the drain must be noted for the calculations. For the wetland itself, volume and BOD removal calculations determine the overall size and dimensions as well.

3. The planting scheme is sourced from the decision on what elements must be removed.

4. The pore size for the media– which serves as rooting material, surface for microbial growth, and filter for particles- must also be guided by the list of pollutants to be removed.

5. Slope is a landscape element which determines the uniformity of flow, and a slope of 0.5% to 1% is generally recommended for construction and proper draining (UN-HABITAT 2008).

6. The present soil conditions determine the kind of sealing the wetland area would require, and also the composition of the soil in the bed.

7. Inlet and outlet structures must be designed to minimize clogging and maximize even flow distribution.

 

SOME IDEAS ON DESIGN

Designs for the wetland could be varied. It could be a single depression lined with impervious material and filled with a substrate and vegetation, or a series of such constructs.

There could also be substrate lining and planting done on the banks along entire length of the channel. This would put up small resistance to the flow, as would a case where the water meandering between islands of miniature wetlands jutting out from the edges of the channel.

A frequently suggested design of redirecting flow from the drains onto the sides at multiple sites is challenging for two reasons, the first being that the cost would be exponential for more than one wetland construction, and the second being that these sites are unlikely to have much area on either side for such constructions outside the channel.

With the Chirag Delhi site, a conventional wetland would be much better given the flatland at the most downstream location of the drain in the settlement. For the IP Estate site, the gradually sloping walls are suggestive of evaluating the idea of the islands of wetlands, but a conventional wetland can be equally prescribed, again where it is widest at its most downstream site after the settlement.

 

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This entry was published on August 3, 2012 at 2:43 pm and is filed under Uncategorized. Bookmark the permalink. Follow any comments here with the RSS feed for this post.

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