Oil and grease effects on biodigestion

  • guilherme
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Oil and grease effects on biodigestion

Dear friends,

I have recently designed a biodigestor to deal with greywater flowing out from a restaurant. During the design phase I had access to a wastewater analysis that showed low O&G content, however, after installation, and to my surprise, O&G are actually up to nearly a 1.000mg/l. It´s leaving the biodigestor with nearly 200mg/l, but the overall efficiency to BOD, COD, TS and others are quite low, as pH nears 5,0. My feeling is that using an appropriate grease trap associated with pH control could enhance the system efficiency.

Questions are:
. Does reducing O&G actually enhance the system efficiency?
. If so, what is an appropriate approach to design a grease trap? All I have seen so far refers to a detention time of nearly 3 minutes - is that enough? My daily flow averages 11m³/day.

Many thanks from Brazil

--
Guilherme Castagna
Fluxus Design Ecológico
Pedra Bela/SP - Brazil

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  • JKMakowka
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Re: Oil and grease effects on biodigestion

It does sound a bit like you are over-feeding the system resulting in overacidification and subsequent inhibition of the methanogenesis. Normally a biogigester will not work well if the pH drops below 6.5.

As grease-trap will remove some of the nutrients and thus might help, but in the end it will also lower gas production if you remove these nutrients.

Have a look at two-stage designs that seperate the hydrolysis and acidogenesis from acetogenesis and methanogenesis phases. The pH is the first stage can be 5-6, while in the second stage it should be 7-8.

Of course, retrofitting an already build system might be difficult, but some sort of buffer tank in front of the digester with some means to control the pH in the actual methane collecting tank does indeed sound like something worth a try.

But I am more talking theoretically here... maybe someone with actual experience building such kind of food waste/ high strength greywater systems could confirm this?

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  • goeco
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Re: Oil and grease effects on biodigestion

Hi Guilherme,
Can we assume that production of methane is the priority, thus a "biodigester"... or do you mean something else by that term? Food grease does have a high potential for production of methane, but I'd imagine would require a long residence time to break down and would best co-digest with other food waste. I'd have thought greywater would be a bit "thin" on organic matter for much methane production.
cheers
Dean

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
www.go-eco.co.nz
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  • guilherme
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Re: Oil and grease effects on biodigestion

Hi Dean, yes my main intention here was to produce biogas with this fixed dome biodigester, question is that I also have to reach a certain quality in the final effluent, thus I may have to compromise biogas production in favour of a lower BOD and COD (if I have to).

My total HRT is 2-3 days on the biodigester, and yes, I do have no other material to digest it with. BOD at the entrance is currently near 2.000 mg/l. O&G near 1.000mg/l. COD near 4.000mg/l. TS ~ 2.400mg/l, whereas volatile solids fraction correspond to 85% of the TS.

Kris, the effluent has very lil to no alkalinity, so I´m ready to dose Na2CO3 right at the entrance to get higher pH and improve the alkalinity buffer. Can you point me out to this two-stage design you mentioned? When you say using a buffer tank before the biodigester you mean something with more storage capacity than a grease trap, right? How bigger?

Best from Brazil,

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Guilherme Castagna
Fluxus Design Ecológico
Pedra Bela/SP - Brazil

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  • JKMakowka
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Re: Oil and grease effects on biodigestion

Can you explain a bit more about the design & design parameters you used?

The relatively low hydraulic retention time lets me guess that we are talking about an UASB ( www.sswm.info/category/implementation-to...tewater-treatments/u ) type digester? Otherwise you will probably never retain a sufficient number of methanogenic bacteria in your reactor.

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  • guilherme
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Re: Oil and grease effects on biodigestion

Hi there Kris,

No, the system I used is a fixed dome digester (as of www.sswm.info/category/implementation-to...atments/anaerobic-di ), but given the circumstances I´m considering a move towards a biogas settler, with baffles as suggested in http://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments/bhttp://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments/b .

You can see the design on the attached image.


I reckon I also have a short-circuiting issue. What do you think?

FYI, I have been producing gas out of it for the past month or so, after I managed to adjust the pH near 6,0.

Best,

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Guilherme Castagna
Fluxus Design Ecológico
Pedra Bela/SP - Brazil

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  • JKMakowka
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Re: Oil and grease effects on biodigestion

I honestly can't give you good advise on this relatively special case (high volume, high strength, but mostly liquid), but a baffled design where only in a later baffle the pH is raised sounds like it is worth a try.

You could also add another baffle with an upflow anaerobic filter to improve the retention of the methanogens, see:
www.sswm.info/content/anaerobic-filter

Edit: maybe using a lime-stone gravel mix for the filter bed would allow you to avoid doing manual dosing to increase the pH?

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