Constructed wetlands need to undergo an automatic process called priming so as to become comparable to a natural wetland. During this process, microbial and various macro-organisms flourish and live in an equilibrium state. Only then it will be able to give the desired results. This process may take some natural time depending on the climate of the area, seasons, etc. The process can be expedite by adding/inoculating vegetation and water collected from natural wetlands.

Thanks Manoj. Are there any complications in sludge retention and management in CWs as compared to conventional WWTPs?

Constructed wetlands will not be effective in treating sludge or solid waste. Sludge treatment requires anaerobic or aerobic digestion processes both of which can be virtually absent in constructed wetlands. Anaerobic digestion requires strict anaerobic condition coupled with suitable elevated temperature whilst aerobic digestion requires sufficient O2.

Constructed wetlands are generally suited as a tertiary treatment system to treat secondary treated wastewater whereby wastewater nitrate-N can be denitrified and DRP can be adsorbed to wetland substrates. With time, the treatment efficiency of DRP will be reduced owing to substrate P saturation. Wetland plants provide conducive subsurface environment to provide biofilm or organic-C required for nutrient assimilation process but wetland plants are poor uptakers of nutrients.

Thanks Selva. Can the sludge handling efficiency be enhanced by combining CWs with waste stabilisation ponds in tropical regions with high ambient temperatures and epileptic power supply for domestic and industrial wastewater. I am looking at decentralised and cheaper wastewater treatment options for rural and suburban areas in developing nations.

Provided heavy metal levels are lower in sludge derived from waste stabilisation ponds, instead of further treating, we need to consider using it as a resource. Sludge is an excellent source of N and organic-C which are beneficial hence could act as an excellent soil conditioner/fertiliser. The issue of pathogens can be dealt by (a) utilising to produce non-food plants (e.g. trees for firewood) or (b) ploughing into soils and leaving it on fallow for pathogen die-off until planting food crops/animal feed. Even if the heavy metals are at high levels, one-off application of sludge is sustainable to improve soil quality.

Significant land area, labour and capital may be required to construct and manage wetland hence you need to consider a simple cost benefit analysis. There is potential for poor performance of wetland plants or even death owing to sludge decomposition process.

Thanks again Selva.

I am not sure if your idea to treat a wastewater sludge by constructed wetlands is correct. I completely agree with answers above. Constructed wetlands are intended for treatment of wastewater with low pollution (COD and BOD) load. They are not intended for treatment of sludges from wastewater treatment, because conditions needed for wastewater sludge stabilization (doesn´t matter if anaerobic or aerobic) can be hardly achieved in constructed wetlands without huge investment to technical design of constructed wetlands.

Regards

Vit

I disagree with the previous answers...constructed wetlands are well known for treatment many different types of effluents, including domestic and industrial wastewater as well as sludge.

They are identified as sustainable treatment technologies, especially for rural communities, since they can be constructed with local materials, have low (or null) energy needs and do not require specialised man power to operate. There are several types of constructed wetlands, you can find information in IWA's Treatment Wetland book (open access): https://iwaponline.com/ebooks/book/330/Treatment-Wetlands?searchresult=1

Dear Bassey,

A very interesting experiment was conducted by Elias Sete Manjate (http://www.bibliotecadigital.ufmg.br/dspace/handle/1843/BUBD-AUEH5W) regarding the application of domestic sludge from septic tanks on vertical flow wetlands (french system). I think this could be a good starting point for your research.

Cheers

After fully reviewing the thesis forwarded by Daniel, I draw the following conclusions about CW efficiency and sustainability in treating sewage sludge:

1. The trials failed to assess full contaminant budget by just assessing sludge contaminant loading and contaminant concentration in the percolate. Assessing contaminant levels in percolate will not report the extent of the treatment since substantial amount of contaminants would have been retained in the beds (Bassey has already posed this question), which will accrue and can contribute to long-term contaminant loading.

2. In general there had been little or no benefit in having wetland plants in terms of contaminant treatment efficiency. This was verified by previous workers in the past, which means by simply having a sludge drying bed is sufficient rather than CWs in most cases.

3. Plant removal of N is only 7.7% of the total-N applied into the wetland. Having reviewed many wetland research papers, this is not a new finding since it is well known wetland plants' uptake of nutrients is low.

4. I am concerned about wetlands constructed without lining since unlined sludge treatment systems can leach substantial amount of nitrate which in turn can pollute local drinking water bores.

5. The word sustainability is widely used hence often abused. In my opinion, sustainable systems must ensure reuse of waste, particularly in countries where there are shortage for resources.

Technically, CWs simply strip organic-C as CO2 (and sometimes as CH4) and strip N as NH3 and N2O, all of which are unsustainable. Why strip tonnes of beneficial nutrients such as ammoniacal-N when this can be used as a fertiliser? Why strip carbon when this can be used to boost soil-C which improves soil conditions and biological activities and increase C sequestration?

6. Other than using as a soil conditioner and fertiliser, there are other sustainable uses for sludge (Bassey already wishes to know the technologies to generate biogas). Judging by the high volatile SS contents in the above thesis study, septic tank sludge can be an ideal source of energy for the local village, since CH4 can be generated by simple anerobic digesters. About 90% of the C can be stripped as CH4 and used and the remaining sludge can be used as a fertiliser.

When dealing with any waste, our philosophy must change. Can the 'waste' be used as a resource? Only when the answer is 'no', we need to consider 'treatment' and 'disposal', both of which are last resorts in the context of sustainability.