Post 3, Week 1

Professor Banzaert's talk with Lyndsay and our Response to the new Specifications

The Sabana Grande already has 2 existing biodigesters. They want another community biodigester, to be tentatively located near/operated by a community leader (who also lives in a dense community). The community members can sell their manure, and then the digester operator/s can sell its gas. 

Since the restaurant digester at La Casita Solar uses a combination of human waste and cow manure as raw materials, we plan to do the same. (we need to confirm with Jorge that the existing digester gets 20 pounds/week manure from cows, and find out if there is much more manure that could be collected). We did the following calculation to figure how many hours of cooking the existing digester can sustain:

• according to http://www.adelaide.edu.au/biogas/poly_digester/aguilar/sld004.htm: 22 kg (50 lbs.) of manure per day gives 12.7 hours of cooking
• The Women of Totogalpa have 20 lbs. of cow manure per WEEK, so 20/7  gives around 3 pounds manure/day. This gives (50/3 = 12.7/x) so x =.76 hours of cooking per day, so 40 mins of cooking per day.

Because 40 minutes is not much cook time, we tried to figure how much human waste would generate a more reasonable amount of daily cooking time:

• according to the book The Truth About Poop, people produce one ounce of poop for each 12 pounds of their body weight.
• So, if you've got a person of 150 lbs, they're pooping 150/12 = 12.5 oz poop per day, which equals .75 lbs/day. And if you've got a person of 75 lbs, they give about .3 lbs/day. People poop between 1-3 times per day, so if everyone in a household (say 2 adults and 2 children) were pooping, that’d give around 2 lbs of poop per day. This gives about .5 hour of cooking time.

If a biodigester got 3 pounds of cow manure a day and 2 lbs. of human waste a day, then there will be enough methane to sustain (40 min. and 30 min.) 70 min. of cooking a day.

We don't think this is enough. And if it isn't enough, then must consider how much will be enough. A digester that is one meter cubed can hold up to 50 lbs. of manure. If 20 lbs of cow waste and 14 pounds (the amount of poop of one family, per week) of human waste a week generates too little gas, then we are talking about building a digester that may be as big as 2 meters cubed.

The digester cannot cost more than $200, either… With this, we proceed with our project statement and initial specs

Project Statement: 

The purpose of our project is to design a plan for the effective implementation of a new community biodigester in the Nicaraguan community of Sabana Grande. The biodigester will consume as much human, animal, and food waste as possible, and convert it to valuable propane that can be sold back to community members. Output waste from the digester will be used as a fertilizer that supports local crops.

Specifications: 

Scale: Digester will be an underground, permanent structure that is larger than 1 meter cubed in size. 

Base scale on output gas (based on how much manure we estimate we will have, and also considering how much gas we imagine the people in the community will need)

Digester Material: Digester will be constructed primarily with local bricks and concrete. 

Base on the local materials in the area, building the most sustainable model that is affordable

Location: Digester will be located near a community leader's home, who will also oversee/operate the digester. It will also be far enough away from homesteads that smell is not an issue.

Specify how far away the digester must be for smell to not be an issue. Also need to find out how discouraging biodigester fuel is, as a result of its stench.

Raw Material for Digester input: Combination of human, cow, and food wastes. The local community is interested in a system that flushes human waste directly to the digester. More than 35 lbs a week total will be needed if we wish to generate more than 70 minutes of cooking time for one family. Note that a digester that is 1 meter cubed in size can only hold 50 lbs. of waste. We therefore likely want a digester that is close to 2 meters cubed in size.

Confirm with Jorge how much manure is being used by the current digester-- is it really 20 lbs?

Cost: Underground digester will cost an initial $300-500. This is more than the $200 budget we have. 

Maintenance: System can clog, and it is hard to clean out. Putting the digester in an ideal location, that has a slope, is essential to allowing the decomposed waste filter out of the digester and be used as fertilizer.

Speculations on Differences between 3 different digester models

Below is a chart we made that compared three different digester models: Underground (based on the Indian model), Plastic Bag, and Floating Drum

	Underground	Plastic Bag	Floating Drum
Efficiency	High efficiency (0)	The dairy digester produced 0.32 m3/day of biogas per m3 of digester volume, 5.5 m3/cow/day. The swine digester produced 0.10 m3/day of biogas per m3 of digester volume per day, 0.15 m3/pig/day. (0)	Low. 20% of gas loss. (-1)
Cost	$300-500 initial (0)	$33-100/ probably more realistic $200 (+1)	550 Euro. $706.64 (in current exchange rate). High material cost for the drum. (-1)
Maintenance	System can clog, nearly impossible to clean out-- must have effective output component for collection of waste for fertilizer (0)	after 3 years need to replace plastic bags completely. (or two years or less-- all depends on plastic quality) Needs to be fed and watered every day. (-1)	Hard. Experts or instructions required. Drums have short lifespan (0)
Installation	Hard, labor intensive (0)	intense. very instructions-intensive, though not that much actual work (0)	Relatively easy. Compact design for household. (+1)
Scale	2 x 2 x 2 m digester requires about 12 lbs manure/ day, yielding 3 meals for family of 4-6 (0)	3x10m, 7.2m^3 volume. produces 1.9m^2 per day,12.7 hours of cooking. need 22 L of manure per day, so 3 cows manure per day (0) 3.5 m long, 1.2 m diameter (520 L liquid) (plastic) = $33 vs PVC ($100)... produces 65 L gas/day.... 80% of the total volume is liquid... (cubic meter of gas will do 3 meals a day for a family of 4-6)... going to produce less than 1/10 a cubic meter a day  excreta produced by 10 fattening pigs will require a biodigester of 4 m³ liquid capacity....	2m^2, 2.5m height. (Dimension of water tank: 1000L. Effective usable volume: 850L) Treats 1-2kg kitchen waste per day. The usable gas volume of the 750L-gasholder is 400L. (-1)
Location	Underground, near pig sty/cow pen (0)	underground, near pig sty/cow pen (0)	Above the ground. (0)
Output (and Time to Produce)	1x1x1 m methane, daily		30Nl (FW) and 103Nl (MW) per day, 8.6% and 109.6% of the measured gas production. 100L biogas=22min burning on cookstove. 2kg kitchen waste = 200L gas. Retention time depends on feedstock (foodstock about 10-40days)
Other  Comments  on Advantage	-underground is best for tropical countries (doi:10.3390/en5082911)The larger scale models, like this one, relate to very large scale factories. This model may be a stepping stone for the construction of bigger bioenergy factory	cheaper than most other types	Constant gas pressure
Other Comments  on Disadvantage (toxic?)	gas can escape through concrete pores, difficult to clean, takes up a lot of space (0)	not very pretty at all, needs to be protected. (-1) a lot of problems with plastic quality
Rank	0	-1	-2

Diagram of Existing Digester in La Casita Solar, from Lyndsay

Find out what community members think of the current digester design-- if it is liked and well understood, then it is probably a good model to go with

Scale-- output

Cost of materials

Location and smell of fuel

Questions for Monday/in Spanish