Environmentally Friendly Waste System

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