Does Polyelectrolyte affect Biogas Production?

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We dewater municipal waste sludge with polyelectrolyte to increase dewaterability.

Does the polyelectrolyte have an effect on the biogas production rate if it is used as feedstock in the anaerobic digester?

Additionally, we dilute the polyelectrolyte with final effluent which is sometimes disinfected by chlorine.

Are the microorganisms in the digester affected by the chlorine content if the residual chlorine=0.25 mg/l and the polyelectrolyte is diluted from 02wt% to 0.05wt% ?

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7 Answers

  1. The situation that you describe is a very normal practice in municipal waste water treatment plants in The Netherlands and Belgium. I worked with rotating belt filters (from the German company Bellmer) and used liquid polymer emulsions from Kemira and Ashland (Praestol). The result was a 5 - 9 % DS slurry. This could be pumped with mono pumps into a sludge digester on site or transported with a vacuum truck to a central sludge digester somewhere else. This process is technically called; thickening with a belt thickener. Dewatering to 19 25 % DS comes after full digestion. 

    If you don't trust the precedence of chlorine then you should feed the Polymer Preparation Units with normal tap water. A well designed sludge handling line should be able to switch between final effluent and fresh tap water. 

    Furthermore: Polymer products for this purpose are in most cases PolyAcrylAmides. They are not easy biodegradable but the will surely decompose into fish smelling rotting products due to the amino groups. 

    1 Comment

    1. Very Interesting. SO does the presence of Poly not lower your Biogas yield based on what you designed for. We do not want to use potable water. Here in SA we work hard to reuse our treated water produced by the plant, so the municipality avoids using potable as much as possible. The remedy would be to replace chlorine dosing with UV treatment.

      But I would like to know how the biogas deviates with poly compare to conditions when not using with poly. I am thinking at initial design stage when sizing the Anaerobic digesters you did not take the the Poly affect into account and only found that out afterwards??

      1 Comment reply

      1. I've never heard that Polymers used in wastewater treatment can affect the production of biogas. Not in a positive way, neither in a negative way. 

         

        What the Dutch water authorities did in recent years is switching from digestion of sludge that was thickened by gravity (1% DS) to digestion of sludge is thickened by rotating belt filters (5 - 7 % DS). In this way they were able to handle much more sludge in their existing digesters or extending the residence time of the sludge in the reactor. In many cases they collected the sludge from different locations via vacuum transport trucks to one central digester. The rotating belt filters were operated unmanned and in most cases the whole wwtp was operated unmanned. 

  2. Not sure I understand your intent so sorry if I have it wrong. You remove solids, (Poly aides removal) then you want to slurry with treated effluent and add to digester? You don't mention what type of digester you have but if this is an IC then there is risk you will wash out, damage the sludge structure and seriously reduce the performance, reducing biogas & increasing discharge COD. On the other hand other constructions can tolerate some level of solids but quantity and nature of solids is important.

    Reducing the efficiency of removal of solids (less poly) or bypassing some flow from solids separation will have a similar effect simplifying your process. Another option is to consider a solids digester - long process time, risk of managing the ammonia and capex.. But some savings in more biogas, less to mass for downstream management.. Have fun..  

    1 Comment

    1. It will be an Anaerobic Digester. At this stage it is an idea to feed this sludge to the Digester but I am worried that Poly in already in the Sludge that will feed the Anaerobic digester might influence biogas yield. I was thinking that the design or sizing of the Digester should be consider the Poly as well...maybe a longer time in the digester.

  3. Normally the polyelectrolyte has very little effect as its concentration is very low. But the chlorine, if it's FREE CHLORINE, it will definetely have an effect. How big effect is to be determined. If it's total chlorine then you should measure the FREE AVAILABLE CHLORINE.It should be below 0,05 mg/lit.

    1 Comment

    1. Thanks for the insight, very interesting regarding poly not having any effect on the yield. I would imagine that it would at least slow the process down a little. Based on the Residual Chlorine amount you have reported, the chlorine will be a problem.

      1 Comment reply

      1. The only chance that polyelectrolyte might slow or even stop the process is if the polymer contains a significant amount of monomer, and if the monomer is toxic.

        Total chlorine is not a problem, only free chlorine.  

  4. Not my area of expertise.  This paper may be of help:

    Preparation of polyelectrolytes for wastewater treatment

    Marilena T Radoiua, , , 

    Diana I Martina, 

    Ioan Calinescub, 

    Horia Iovub

    a National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerator Laboratory, P.O. Box MG-36 Magurele, Bucharest R76900, Romania

    b Industrial Chemistry Faculty, Polytechnic University of Bucharest, Calea Victoriei 149, Bucharest R71101, Romania

    Received 14 April 2003, Revised 24 July 2003, Accepted 5 August 2003, Available online 15 October 2003

    http://dx.doi.org/10.1016/j.jhazmat.2003.08.014

    Abstract

    Liquid-phase polymerisation of acrylamide-acrylic acid to form polyelectrolytes used in wastewater cleaning was examined using accelerated electron beam and microwave irradiation methods. Polymerisation was carried out in aqueous solutions at temperatures ∼60 °C. Monomers total concentration was established at 40% (36% acrylamide and 4% acrylic acid). Only using the features of simultaneous radiation–induction and microwave heating can result in the formation of linear polymer chains with good water solubility and low residual monomer concentration. The flocculation capacity of the obtained polymers was tested using two wastewaters, one sampled from a slaughterhouse and the other from a vegetable oil plant. Quality indicators such as total suspended matters (TSM), chemical oxygen demand (COD), biological oxygen demand (BOD) and fat, oils and grease (FOG) were measured before and after the treatment with polymeric flocculants and compared with the results obtained in classical treatment with Al2(SO4)3. It was found that the combined treatment with polymers and Al2(SO4)3increases the degree of purification of both wastewaters up to 99%.

    Keywords

    • Radiation-induced copolymerisation; 
    • Acrylamide polymers; 
    • Electron beams; 
    • Microwaves; 
    • Wastewater treatment

  5. My master thesis was about anaerobic digestion, and during my reserach I couldn't find these kind of specific researches related with some type of inhibition/interfernece with substances like those you mention; anyhow, I saw that anaerobic digestion is a very sensitive process, so I could say that even if there's no inhibition, the content of those material could lead to a detriment in the performance of the system. It's better to spend a bit of time and money with some pilot experiments rather than find a fail during operation. Good luck! 

    2 Comments

    1. Our local water and sewage authority has been using anaerobic digestion since 1991. The polyelectrolyte they are using poses no problem. They actually produce BIOGAS to heat up the building.

      1 Comment reply

  6. Interesting questions, but why don't you add the polymer after digestion? Phrased another way, what benefits are you expecting to see from adding the polymer to the digester feed instead of dosing the polymer into the digested sludge just before dewatering?

    1 Comment

    1. The dewatered sludge is actually transported to a landfill site. Here in SA we pay per volume sludge transported. To reduce the volume we dewater. Poly is added to increase the dry cake solids by aiding separation of water and sludge. The feasibility will be investigated in the futerture of making a central Biogas facility and instead of transporting sludge to landfill, it is transported to the facility. However, this sludge will contain poly and the poly is diluted with chlorine treated water. The feed to the digester will therefore have poly and this cannot be changed. I have asked this question to many at seminars but none that I spoke to have researched this parameter.

      2 Comment replies

      1. Thank you for clarifying, this makes more sense. I agree with John that you shouldn't have to worry about the chlorine, because the small residuals affect on the digester sludge will be negligible.

        I too have not heard of any studies into the effect polymer has on biogas production. Here in the US, we often thicken sludge prior to anaerobic digestion to increase the solids throughput capacity of the digesters. The thickening processes include polymers. Like I said above, I haven't heard of any studies on the effect polymer has on anaerobic degradation, but I would venture to say that there isn't a large enough effect for anyone to take notice and decide to run some pilots. If you do run any tests, I'd be interested in seeing the results. 

        Sounds like you may be thinking of a dry anaerobic digestion technology for the biogas facility if you will be feeding dewatered sludge. If the sludge isn't hydrolysed prior to a liquid anaerobic digester, the solids feed would be capped around 5-7%, due to pumping and mixing limitations. I'd guess you currently dewater sludge to 15-30% solids. 

        Sounds like a cool project, where is the project located in South Africa?

      2. I'm assuming you can't build the biogas facility next to you WWTP, correct?  Otherwise you would just be able to pipe it directly from the digester and bypass your cost to belt press and transport.   But to answer your first question, NO it would not hurt the biomass simply because by the time is enters the system the chlorine would have either been used up or gassed off.  THe demand of the sludge would be so great it would overcome any effect from a 0.25 mg/l concentration which is close to drinking water standards at the tap.

  7. Not if  used correctly

    2 Comments

    1. Commonly used polyelectrolytes (cationic) for sewage sludge conditioning prior to dewatering normally  do not inhibit water uptake of the dried sludge.

      The BOD/COD ratios of the conditioned sludges also  show that the biological degradability of the sludge is not adversely affected by the sludge conditioners. However, as suggested above. by Mr Thomas, a more effective way could be adding the PE's after digestion. Of course one may have to try your approach in case your primary is hydraulically overloaded.  however, please do carry out a lab scale study first with your PE and dosage to be doubly sure in view of Mr Romero's comments.