[ghazala]

HI evry1,

i'm doing a design project on anaerobic digestion of MSW and i'm having problems with choosing an equation to calculate the size of the digester. i've come across 3 equations. here are the results obtained:

                 % Composition
                   (by weight)   Moisture %   Volatile Solids %
Yard waste              57          60               70
Kitchen waste          31          70               92.4
Paper waste            12            6               94.8

this data was given in the design statement. i'm supposed to treat 32 tons/day of MSW and with the system i'm using my retention time is 20 days.

1st method:


                                  Mass (kg)                   Moisture (kg)
Yard waste           57/100 * 32 = 18.24      60/100 * 18.24 = 10.944
Kitchen waste       31/100 * 32 = 9.92        70/100 * 9.92 = 6.944
Paper waste         12/100 * 32 = 3.84          6/100 * 3.84 = 0.2304

i find the moisture content of each component.
the total moisture content = 18.1184 ton/day

Density of water = 1000 kg/m3
therefore, 1 ton = 1 m3

volume of digester = volume of feed (m3/d) * retention time
volume of digester = 18.1184 * 20
= 362.368 m3

2nd Method

feed flow rate = 32 ton/day

V = HRT * Q ( p46 - biomethanization of the OFMSW)

V = 20 * 32
= 640 m3

3rd Method

Bulk Density of MSW in Mauritius = 200 - 224 kg/,m3
taking bulk density to be 224kg/m3,

1kg = 1/224 m3


                                    Mass (kg)              Dry matter (kg)
Yard waste              57/100 * 32 = 18.24   40/100 * 18.24 = 7.296
Kitchen waste          31/100 * 32 = 9.92     30/100 * 9.92 = 2.976
Paper waste            12/100 * 32 = 3.84     94/100 * 3.84 = 3.6096

total dry matter = 1388.16 kg/d

volume of digester = feed in m3/d * retention time
volume of digester = 1388.16/224 * 20
= 1239 m3.

These are the 3 methods i used, i would be grateful if anyone could guide me through the choice of the equation to be used.

thanks.

[eugenedakin]

Hello ghazala,

Good question, and lets work through each scenario.

If you are required to treat 32 tons/day at 20 day retention time, then you must have a minimum of (32 x 20) 640 tons (640 m³) per 20 day cycle.

Answer #1 is ~ 363 m³. This will not meet the minimum requirements, so it is eliminated.

Answer #2 is 640 m³. This meets the minimum requirements, but does not allow for system poisoning, or tank maintenance. The built up solids will need to be removed at some time, so you will need more than 640 m³.

Answer #3 is 1239 m³. This provides a safety factor of about 2. If the system becomes poisoned, then you have storage to regenerate new microorganisms and/or slowly dilute the poisoned system. To be on the safe side, I would choose #3.

I hope this helps.

Sincerely,

Eugene

[ghazala]

Thank you so much Eugene, its been a great help

[ghazala]

Dear Eugene,

I am thinking of using a process similar to that of the dranco process where the waste will recirculate through the digester on a basis of four days, so this will prevent poisoning. if i take the 640 m3 digester and account for some space, say 10% more for the biogas produced, will it be ok?

Regards,

Ghazala.

[eugenedakin]

Hello Ghazala,

I don't know if you can prevent poisoning. If you can, then this would be great! I thought that the Dranco system needed about 20 days, and the bacteria were thermophilic? The energy from the gasses from anaerobic digestion are used to power natural gas engines, and the excess heat is used to increase microbial action. I am not sure how 10% more space in a 4 day digester could help.

Could you help me understand this a little more? Thanks for your patience.

Sincerely,

Eugene

[ghazala]

hello Eugene,

Yes, the Dranco system does have a retention time of 20 days but it also recirculates the digestate 6-7 times in the digester. it uses the gas to produce electricity and heat. i need to design such a plant. increasing the volume by 10% would give space for accumulation of gas in the digester.

Regards,
Ghazala.

[eugenedakin]

Hello ghazala,

You have provided some interesting numbers. I have found an article which provides some values:

http://www.ows.be/pub/The%20Dranco%20Technology%202006_alg.14634%20Rev00.pdf

A quote from this article states:

A single digester of 3150 m³ of total volume can treat 50,000 ton per year of organic waste

This seems as if these values would be the 'best case' - and would assume no poisoning or low nutrient feedstock.

50,000 tons per year = 137 tons per day with a digester of 3150 m³

If your system is working perfect, then assuming these above numbers from the article are correct, your 32 ton per day system would require about 736 m³. This would fit with your additional 10% suggestion.

Since operators are human, and we all make mistakes, would you be able to have a fully trained staff, good feedstock, and no problems with poisoning or contamination to make sure this optimal system works?

This is up to you to decide. If the system does not work as planned, then you would need to justify your actions.

I wish you the best with your project.

Sincerely,

Eugene

[ghazala]

Dear Eugene,

thank you very much for taking the time to reply to my post. This has helped me a lot.

Kind Regards,

Ghazala

[kbarry]

Hi I am trying to find some information on a small to mid sized version of an anaerobic digester.  Our cafeteria produces a little over a ton of food waste per week.  How large of vat would be needed for that level of flow?  Also what would be a good retention rate for producing a steady amount of methane?  The methane can be used directly for steam production for the school's steam network so its production is the priority.  If you have answers or can direct me on where to find further information it would be much appreciated.

[ghazala]

Hi kbarry,

well for the volume of the disgester u'll need to find the density of the waste, then calculate its volume as follows:

volume of digester = volume of feed/day * retention time

volume of feed = mass of feed/density of feed

Now, for the retention time, if u plan on using thermophilic conditions ( i.e heating the digester up to 55 degrees celcisus) then u'll have a retention time of 22 days in the digester, whereas if u do not plan on heating it, using mesophilic conditions, u'll have a retention time of around 40 - 50 days.

Regards,

Ghazala

[tarun2003]

Hi,
   If I were to setup an aneraobic digester locally that gets its feed everyday, how do I make sure it is air-tight at the time the feedstock is being fed to it?