Primary Chemical treatment of industrial effluent prior to biological process: A solution turned into acute problem

Most of the industrial wastewater treatment plants utilizing biological processes are always designed with a primary treatment incorporating various chemical addition. The primary goal of such primary chemical treatment is to remove pollutants using chemical treatment and subsequent precipitation. Our recent surveys of some of the industrial wastewater treatment facilities having both primary chemical treatment and secondary biological step had provided us some useful but shocking insights in the practices We had observed at the plants. Most of the plants were adding huge quantities of chemicals in the pre treatment even without knowing the basic aim of adding such chemicals. Upon asking what were aims of adding such chemicals and which pollutants were the target pollutants to be removed at the primary stage, most of the clients were clueless. They had just one common notion that prior to biological removing as much “load” as possible would be helpful. They were under impression that adding such chemicals at higher quantities would remove more pollutants and thus stabilize their biological process better.

On the contrary, it was found that such reckless addition of chemicals in the primary treatment had altered the chemical properties of the liquid so badly that ultimately the performance of the activated sludge was severely affected resulting in poor removal efficiencies at the biological stage.

This reckless addition of chemicals in the primary treatment may create lot of issues with the downstream biological process such as:

  • It may increase temperature which can affect the biological process changing the growth rate of microorganisms and even affecting the oxygen transfer efficiencies of diffused aeration system.
  • It can increase the inert or inorganic fraction of the MLSS and thus for the same level of MLSS maintained, it reduces the MLVSS/MLSS ratio causing lower MCRT and may cause poor performance.
  • If wasting is not practiced correctly then it can lead towards building up of this inorganic fraction and associated toxicity within the MLSS causing further adverse effect on the biological process performance.
  • Due to change in osmotic pressure under severe conditions, it can reduce the uptake of organics which are targeted to be removed during biological step.
  • It can alter the zeta potential of the MLSS containing liquid so badly that floc formation becomes very difficult causing deflocculation and wash out of useful, active biomass via TSS carryover in the secondary clarifier outlet. This may cause severe performance loss if not controlled well enough.
  • Some of the chemicals added for primary treatment have capability to react with PO4 added for right COD:N:P balance and reducing its availability to microorganisms. This may result in poor COD reduction, poor flocculation properties and reduced biomass growth. 

There is also possibility that the added chemicals may form specific complex molecules with some of the organic substances which otherwise are not consumed by the microorganisms for their growth. This unnecessary reaction of these chemicals with biodegradable organic substances also results in reduced bioavailability of these substances. These organic substances may get carried over in the treated effluent resulting in higher COD then anticipated.     

To all our colleagues who are operating an industrial wastewater treatment facility comprising chemical primary treatment as well as activated sludge or any biological processes like MBBR or SBR, we would suggest few simple steps to consider while working on the chemical primary treatment.

  • Study your inlet wastewater chemical properties well.
  • Find out which is the ideal growth condition for your biological step and work out which specific organic and inorganic pollutants present in your inlet wastewater might affect the biological performance. As for any effluent treatment process which involves biological step, we need to understand that the biological step is the most critical process and thus any process or unit operation being carried out in the whole process train must be adjusted with reference to the biological process step. It is imperative that We follow this principle rule while establishing the chemical pre treatment for better results at biological step.
  • Establish your chemical treatment targets. Focus on removing only those inorganic and organic pollutants in the chemical treatment step which pose a threat to microorganisms. If any specific pollutant which otherwise not harmful to microorganisms but still required to be removed from wastewater then target their removal after biological treatment as post treatment. This will help to avoid unnecessary altercations of chemistry prior to biological process and thus will help microorganisms to grow well. From chemical process point of view also this approach is economical because most of the chemical treatment are nonselective and thus may also interact with other interfering substances without letting us know about it.            
  • Try to understand ideal operating pH range for target pollutant removal and precipitation ability of the formed complex. Many of the plant operators are just adding chemicals as per the specifications given by suppliers without even thinking about this operating pH range and precipitation ability. If after addition of chemicals, flocs are not formed then the chemical complex formed would be entering to the biological stage causing further issues.
  • Make jar testing a habit for the adjustment of chemicals and keep doing it as frequently as possible shift wise. At many plants perhaps once in a week or once in a day jar testing are conducted. It is rare that any industrial production facilities will have constant flow of wastewater in terms of its chemistry. It will be varying perhaps on hourly basis and daily basis. So based on these variations, the chemical dosing adjustment is must to get better and optimum results otherwise the chemicals may be wasted away without giving any results at all.

Stop using spent chemical obtained as by product from other chemical processes like spent caustic, acid, PAC etc. These spent chemicals contain a lot of harmful impurities in it which in fact add more toxicity to the effluent rather than removing any specific target pollutants.           

We help industries to optimize their chemical treatment as well as biological process offering complete process optimization solution for their complex effluent treatment needs. If you have an ETP and facing problems in achieving required results, then get in touch with us on We are always happy to hear from the clients.