If you want to affect the BOD5 reading as little as possible I recommend to use NaOH or Bicarbonate for increasing the pH since if you use Lime you risk to have Precipitation. Do not forget to take the resulting dilution into account in you use diluted NaOH

If you want to affect the BOD5 reading as little as possible I recommend to use NaOH or Bicarbonate for increasing the pH since if you use Lime you risk to have Precipitation. Do not forget to take the resulting dilution into account in you use diluted NaOH

Effluent high in volatile organic acids (TVOA's) is prone to rapid acidogenesis which lowers pH. I have experience with anaerobic systems that combine acido- and aceto- with methanogenesis for reduction of BOD/COD and conversion of OM to biogas in heavily loaded effluent. Most systems use NaOH to adjust pH as mentioned above. At pH 3.5 microbial degradation is nearly halted. I speculate that your effluent originated at a higher pH which was subsequently altered by microbial degradation. This would indicate that your pH 3.5 sample has lost some BOD in the process. Samples are time sensitive, must be immediately chilled, and assayed within hours of collection. 

As to manage the pH,   buffer tablets/liquids are widely available in market. With this pH can be made to desire value .

I think its better to use a buffer solution such as (phosphate buffer) to increase and maintain pH level.