COD reflects the amount of organic matter and mineral matter, either dissolved or suspended in the industrial effluent.
What is the plant output?
What are the average monthly COD levels?
Is there a seasonal factor?
There is a textile factory upstream of our WWTP, and overall production is constant throughout the year.
You should look at the ratio of COD to BOD5 in your influent to see how it changes.
Ideally, you would want to collect a series of equal volume grab samples over a period of 24 hours to try to pinpoint the peak and then conduct further analyses based on these time frames.
This would also enable you to see whether the increase occurs at regular intervals.
Production within the industry is based on production schedules, so they know exactly which product they were producing at a certain day and time. Contact the textile plant (or any other industry) and ask them what they were producing during the day, how long and how much? Thus you might be able to identify the product which causes or caused the high COD levels.
Textile wastewater shows somestimes a very low biodegradability. Check the COD/BOD ratio if still possible and if there is an increase in the ration this would be a further indication.