Making the Case for Sewage Sludge Treatment via Pyrolysis

[NNadir]

The paper to which I'll refer in this brief post, a "perspective" as opposed to a research paper, is this one: Pyrolysis Solves the Issue of Organic Contaminants in Sewage Sludge while Retaining Carbon—Making the Case for Sewage Sludge Treatment via Pyrolysis (Wolfram Buss ACS Sustainable Chemistry & Engineering 2021 9 (30), 10048-10053).

I spend a lot of time thinking about sewage sludge because human fecal waste, even treated sewage sludge as well as untreated fecal matter, is a major contributor to morbidity and mortality on this planet.

An excellent and fascinating book, which I read quite a number of years ago on this topic - it is nontechnical and for popular audiences - is The Big Necessity: The Unmentionable World of Human Waste and Why It Matters by Rose George. I recommend it highly to anyone interested in a sustainable world and justice.

In addition the World Health Organization has several websites devoted to the death toll associated with untreated fecal waste.

Many toxic compounds have found their way into treated sewage sludge as a result of pollution, the use of "personal care" products, pharmaceuticals and contaminants in foods. If sewage sludge is used as a fertilizer, seriously toxic compounds are subject to uptake by plants.

However a critical component for the survival of agriculture, phosphorous, currently obtained by mining and thus subject to depletion, is found in significant concentrations and thus represents an opportunity for some level of circularity.

I am a long term advocate of pyrolytic treatment of organic wastes, and was pleased to come across this nice review with lots of references.

From the text:

Hundreds of millions of tonnes of sewage sludge are produced globally each year.(1) Historically, sewage sludge has been applied to agricultural land to recycle nutrients and carbon.(2) However, land application is a growing concern as insufficiently treated sewage sludge contains hundreds of potentially harmful organic compounds, many of which are unregulated.(3−6) Such emerging contaminants, e.g. personal care products and pharmaceuticals, can be taken up by plants and enter the human food chain.(7,8)

Composting and anaerobic digestion are unable to sufficiently remove or degrade organic contaminants from wastewater solids for safe recycling.(9−11) Incineration oxidizes and degrades contaminants but also results in loss of carbon and some nutrients.(12) Landfilling of sewage sludge is already banned in many countries in the EU.(13) Therefore, with mandatory recovery of phosphorus from wastewater now in legislation in Europe,(14−16) innovation in sewage sludge treatment is urgently needed.

Heating sewage sludge at elevated temperatures (∼300–800 °C) in an oxygen-free atmosphere in a process called pyrolysis, converts carbon in the feedstock material into a form stable for centuries(17) and retains potassium, calcium, magnesium, and phosphorus.(18) Furthermore, recently it was demonstrated that spiking sewage sludge with low levels of potassium (2–5%) prior to pyrolysis can make–the typically unavailable–phosphorus water-extractable and hence plant-available.(21) The benefits of using sewage sludge biochar in agriculture for soil improvement and nutrient provision have been highlighted in several reviews.(2,19,20)

Incineration degrades organic contaminants with high efficiency and is the method of choice for removal of organic contaminants from solid materials.(12,22,23) Pyrolysis, however, is not widely accepted as a method for contaminant removal from sewage sludge. In the recent Joint Research Centre (JRC) report (public consultation ended 02/2021) “Technical proposals for selected new fertilising materials under the Fertilising Products Regulation (Regulation (EU) 2019/1009)” by the European Commission (STRUBIAS report), input materials for pyrolysis eligible for the production of fertilizers in the European Union were defined.(24) Sewage sludge was excluded as a feedstock material in the report since “the knowledge base of studies that assessed the proportional removal of specific organic pollutants is limited and restricted to only a few organic pollutants”,(24) and the authors recommended additional research.

In the current study, I evaluate the available evidence on removal of organic contaminants from sewage sludge via pyrolysis (summarized in Table 1) and compare the environmental implications of pyrolysis treatment with incineration. I conclude that the data are supporting pyrolysis as a technology to produce safe sewage sludge-derived biochar fertilizer...

A table from the text showing data collected from various publications on the effectiveness of heat in the destruction of toxins found in sewage:




The author continues:

Besides organic contaminants, the content of potentially toxic elements (PTEs) can be elevated in sewage sludge and pyrolysis enriches PTEs in the biochar.(41,54) Yet pyrolysis also immobilizes PTEs, which reduces soil leaching and plant availability. Therefore, with threshold values for total PTEs in sewage sludge biochar in place, there is no risk for land application.(55−57)

As clearly outlined in this manuscript, pyrolysis of sewage sludge removes and destroys organic contaminants. While results from small-scale batch units cannot always be transferred to large-scale, commercial production units, data reported in this manuscript are based on studies that utilized a variety of pyrolysis unit of different scale. Furthermore, previous work demonstrated that key biochar properties were comparable in biochars produced with small, lab-scale batch units and pilot-scale, continuous units.(58,59)...

...and later...

With increasing concern over emerging contaminants in sewage sludge and a need to recycle P, thermal treatment of all the sewage sludge generated globally seems unavoidable. Incineration and pyrolysis are both technologies that show high efficacy in removing organic contaminants, and in contrast to landfilling, both give the option to recycle the sewage sludge-inherent-P in agriculture. Since there is a need for greenhouse gas emission reduction and even atmospheric carbon capture and storage,(62) energy requirements and carbon emissions are the next essential criteria for choosing the most suitable technology for sewage sludge treatment.

The moisture content of sewage sludge and biosolids (treated sewage sludge) can be >95%,(63) and drying prior to thermal treatment is necessary. Drying is a very energy intensive step—it can consume 5-times more energy than is needed for the pyrolysis process—therefore the treatment of wet sewage sludge via pyrolysis requires external energy addition.(63−66)

If the energy is produced using dangerous fossil fuels, the energy for this process would drive climate change as much as the failure of so called "renewable energy" is driving it.

Many of my considerations in recent years however have been addressed at heat networks driven by nuclear heat. It is clear that in this case, pyrolysis could go a long way to solving a critical health and environmental risk.

Have a nice weekend.

[OAITW r.2.0]

1. Better living thru Chemistry!

Makes sense to utilize chemical processes that recover elements while neutralizing the waste stream.

[John ONeill]

2. Too much water

Vacuum sewerage systems use much less water than the usual ones - they would cut the amount of energy needed for drying. Backfitting would be expensive though. An engineer friend of mine was telling me about a system for feeding a liner into old underground pipes, coiling it till it forms a helix, then pulling a wire on the edge that rips open an epoxy-type sealer, sealing the liner. The narrower bore is compensated for by the liner having less friction than the original pipe. Not sure if the system could handle a vacuum though. He says it's used as it's about half the cost of putting in a whole new pipe, and should last about three quarters as long.

[NNadir]

3. I see it somewhat differently.

Water raised to the temperatures at which these pyrolysis events take place is in a supercritical state and thus under these conditions is an oxidant of organic species, both toxic and non toxic. The water is reduced to hydrogen gas and carbon oxides, basically syn gas, presumably with a residue, depending on conditions, or pyrolytic carbon, itself a valuable material.

I was unfamiliar with the claim about potassium additives converting pyrolytic carbon into slow release phosphates, but I will get around to calling up the reference.

This review did not explicitly cover the issue of pyrolytic gases, but these are of significant value as well. There are many papers discussing this topic.

My ideas on nuclear energy are designed to make use of the high temperatures at which fission occurs, connected to a large heat network. The supercritical water/pyrolytic system could be a side product, or rather an additional product, of that heat network, with the production of electricity also being a side product.

The issue is to close the carbon waste cycle, and the chemical waste cycle while providing clean water and clean air.

[Wicked Blue]

4. How do you think this might affect PFAs?

And what do you think could be done about heavy metals in the sludge?

This reminds me a bit of an EPA project developed in New Jersey in the 1980s using high heat incineration to get rid of toxic leachate from landfills. They built a mobile incinerator nicknamed the Fire Dragon and tested it at the Kin-Buc Landfill in Edison NJ.

https://nepis.epa.gov/Exe/ZyNET.exe/9100GQZT.txt?ZyActionD=ZyDocument&Client=EPA&Index=1981%20Thru%201985&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C81THRU85%5CTXT%5C00000017%5C9100GQZT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=5

I worked at the Star-Ledger covering Middlesex County at the time and wrote several articles about this project.

[NNadir]

5. Ref. 36 in the table from this review article refers to PFAS.

I haven't had a chance to access the paper yet, but here's the reference:

(36) Kundu, S. K.; Patel, S.; Halder, P.; Patel, T.; Hedayati Marzbali, M.; Pramanik, B. K.; Paz-Ferreiro, J.; de Figueiredo, C. C.; Bergmann, D.; Surapaneni, A. Removal of PFAS from Biosolids by a Semi-Pilot Scale Pyrolysis Reactor and the Application of Biosolids Derived Biochar for the Removal of PFAS from Contaminated Water. Environ. Sci. Water Res. Technol. 2021, 7, 638.

This seems to be an adsorption paper however.

I have written in this space about decomposition of PFAS, an issue that is on my mind quite a bit. Here's a relatively recent example.

A Nice Scientific Review Article on the Destruction of Persistant Perfluoroorganic Pollutants.

As for me, I think I have a hammer, and thus every problem is a nail, I favor radiolysis as an approach both for water and air (CFCs HFCs) for destroying fluorinated compounds.