pH limits for sanitary and combined sewers discharge set by City of Toronto, Canada is  between 6.0 to 11.5. As you are receiving at close to 7 is not a problem. Your problem is change in pH.

Some microorganisms are more prone to change in environmental conditions such as , light, temperature, pH and redox potential. So, check how you are  storing your wastewater stock. Store  it  at 4 oC, cover in black (protection from light). If you have conducted biological  analysis you may know which microorganism are still active. Elemental analysis may tell you if existing contents are reacting. 

If your system allows, you  may  add a pH supplementation (such as magnesium hydroxide Mg(OH)2 slurry. 

Lorenza

The pH changes observed resulted primarily from natural, aerobic metabolic processes in wastewater. These microbial reactions form byproducts like formic, acetic and other common organic acids which form from abundant effluent DOCs.  Municipal wastewaters are typically highly buffered by bicarbonates and carbonates which ironically are also byproducts of aerobic biological digestion like those in the more familiar fermentation processes in wine making.

http://www.emsb.qc.ca/laurenhill/science/wine.html

Steve

Hello,

thanks for your answers. Yes actually I was thinking about fermentation processes by microorganisms in the wastewater. Keeping the batch in the cold room does not help in the long run. Do you think I could freeze the wastewater to preserve it?

I agree, you most probably have a starting fermentation process. - actually storing wastewater over months is usually not such a good idea. - We only do it for very stable compounds.

pH of waste water in the begging was neutral ( 7.0 ) and storing these samples for long time like 2 months .obviously, the undissolved constituents which may include undissolved gases may be reacted to dissolution process under chemical kinetic condition.Result of this pH may be effected and become 7 to 5.

Lorenza

I checked with our microbiologist to see whether there were any acceptable long termed municipal wastewater storage practice or recommendations.  The suggestion was that biologically active samples should not  be kept for more than 24 hours even with refrigeration.

Unless you wanted to quench any biological activity by adding anti bacterial agents or perform sample sterilization  by using an autoclave.

Steve

I say freeze it... you filter away whatever microorganisms are in there with the 25 um filter anyway right? At least that is what assume that is what that this step is for.

Also I am assuming that it is not viable to autoclave since you run the risk of having some compounds degrade.

Then store it at -80... then you could probably keep it as long as you want. And you can always test with a small aliquot to see if the pH changed.

Dear 

it is clear the values of pH change with time, this is the reason to conduct temperature, pH ORP ,and  Do . all this parameters are affected  my the activity of microbes and the temperature of the waste corresponding with the ambiance temperature.  i do not know ether your sample is affected by adding of preservative chemical but i advise you just measure pH on the spot.  

the results of pH change is the aerobic metabolic processes in your wastewater. which can form weakly oraganic acid as a results of degredation of Organic compounds present in your water.