Environmentally Friendly Waste System

We have wanted to run our first run of the experiment for quite sometime, however, due to circumstances, we have not been able to run the experiment proper.


We met with difficulties when the coupling of the vacuum truck and the digester did not fit, also he second time we went down to the SVF, we realised that there was no sludge available nor will there be any available anytime soon. This was due to the camps being cancelled in view of the H1N1 situation. After much discussion, we took Dr Han's idea of using municipal waste instead.




In summary, these are the problems that we have faced:


1. Lack of planning for our first experimental run; not fully prepared
2. Coupling of the vacuum truck and the digester did not fit
3. Cancellation of camps due to H1N1 situation
4. No sludge is available at Singapore Vision Farm (SVF)




Classification of various causes leading to the main concern of not able to carry out the first experimental run is done using a cause and effect analysis.

Furthermore, in this project, we are trying to collect the methane gas produced during the anaerobic stage of digestion. That is the main focus at the moment. Hence, although municipal waste is used instead of human waste, methane gas will still be produced and the collection can be done. The only problem is that we still cannot prove that the human waste that is digested is fit to use as fertilizers for plants.



In addition, since we are also thinking of introducing a wetland system, it doesn't really matter what kind of sludge is used. The only concern may be the extra contaminants contained in the municipal waste that needs to be properly dealt with.



Right now, we are doing calculations on the sizing of the methane collector. Our idea is to use the solubility of the other gases like CO2 and H2S and calculate that in relation to the residence time. From the residence time, we can gauge the height of the tank needed. The initial thought of design will is a thin but tall tank, that way, the other gases can be dissolved more easily as compared to a larger tank. This is because the bubbles that come through the tank will only be in a straight line and hence there isn't a need for a large tank.



More on the calculation will be updated

Prior to our first experiment, we have listed out the various parameters that we will need to take note of for digester operation.




Optimum environmental conditions for methanogens

• Optimum temperature: Mesophilic range- 85oF ~ 100oF

• Recommended pH range: 6.6~ 7.6 (beyond this limits, digestion will proceed with lower efficiency) At pH values <6.2, the digester efficiency drops rapidly and the acidic conditions can become damaging towards the methane bacteria.

• Optimum pH: 7.0~ 7.2

• Biological nutrients required:

o nitrogen

o Phosphorous and others

• Anaerobic conditions: small amounts of Oxygen can be detrimental to the methane-formers hence requiring a closed digestion tank.

• Free from toxic materials accumulation of heavy metals and inhibitory organic and organic compounds.



Indicators of unbalanced treatment

• Increasing Volatile acid concentration (best and first indicator of digester unbalance) increase due to slower utilization of volatile acids by methane formers.

• Increasing CO2 concentration (percentage) in gas  unbalanced digester treatment usually results in the decrease in methane formation which leads to an increase in Carbon dioxide percentage.

• Decreasing pH (results from the high volatile acid concentration) significant drop in pH does not occur unless digester is “stuck” (zero efficiency)

• Decreasing total gas production (first indication due to toxicity)  useful as a parameter if the daily feed intake is uniform and the gas production does not vary too much day to day under normal conditions)

• Decreasing Waste Stabilization drop in methane production per pound of volatile solids added indicates a drop in operation efficiency / increase in COD



Factors causing unbalanced (lowered operation efficiency) treatment

• Temporary Unbalance can be corrected by providing optimum environmental conditions and controlling pH.

o Sudden change in Temperature

o Sudden change in Organic Loading

o Sudden change in nature of waste

• Prolonged Unbalance

o Presence of toxic materials (pH control will not solve the solution)

o Extreme drop in pH

o Slow bacterial growth during start-up.





Control of Treatment Unbalance

First step: to control pH

Second step: Determine Cause of unbalance look into temporary and prolonged causes

Third step: Correct cause of unbalance

Fourth step: Provide pH control until treatment returns to normal

Note: unbalance is accompanied by high concentration of volatile acids.

pH control

-decreasing waste feed to digester

-neutralizing the waste with lime (cheaper) or sodium bicarbonate