High rate mixing system shall reduce the sludge generation

I think that the aerobic reactor operating in extended aeration is a alternative.

I agree ozonation may be expensive. Nevertheless it should not be discarded a-priori: it may be a viable option if an ozonation unit is already present in the plant for tertiary treatment to remove color and surfactant and achieve disinfection, too. Payback period in one plant (110.000 PE, 80% of COD load coming from textile wastewater) has been calculated as less than 3 years. Ultrasounds are even more expensive than ozone and cannot be used for tertiary treatment. A more convenient method is thermal lysis (Cambi or Biothelis processes) placed before anaerobic digestion of sludge: it enhances biogas production (and recovers the extra energy needed for the process) and reduces the amount of sludge produced. I think that a process based on the concept of the OSA process is the Cannibal(R) process (marketed by Siemens Water Technology). Sludge reduction efficiency highly depends on wastewater and process characteristics.

@Enrique, Thanks for the reply. I am unable to open the attached .pdf (tryed many methods). Do you have a dircet address to the pdf or another path to the document?

Thanks.

David

A cheap process change, which I suppose it could work for you is Moving Bed Biological Reactor. There is still not much literature on this, but you can read the following papers:

Luigi Falletti and Lino Conte, "Upgrading of Activated Sludge Wastewater Treatment Plants with Hybrid Moving-Bed Biofilm Reactors" - Ind. Eng. Chem. Res. 2007, 46, 6656-6660.

Giorgio Mannina and Gaspare Viviani, "Hybrid moving bed biofilm reactors: an effective solution for upgrading a large wastewater treatment plant", Water Science & Technology 2009, Vol 60 No 5 pp 1103–1116.

I agree with Prof. Canziani that a convenient method is thermal lysis before anaerobic digestion of sludge. The Cannibal process is capable of reducing excess

sludge up to 60% compared to conventional AS, but the bottleneck seems to be the solids separation module. The HRT and the interchange rate are important parameters for achieving the required target of solids destruction.

Thanks for all your answers.

David.

What exactly is your Q? Do you mean producton of less excess sludge? Otherwise, more sludge in the reactor the better the results ... Compare to this parallel: More guests on party, more food will be eaten, - or with same food available, more guests will leave less uneaten ... Make sound?

@ Boris: Sludge digestion treatment is a tertiary stabilisation of excess sludge from the A/S process. The question is what methods are applicable during secondary treatment to reduce sludge volume for tertiary treatment YES excess waste activtaed sludge. i.e Your party aanalogy alludes to metabolic pathways...uncoupling of metabolism such as occurs through use of MBRs, predation on bacteria by protozoa, etc. Techniques that exploit the biological/biochemical pathways employed by bacteria. MORE CATABOLISM less METABOLISM.

Thanks Boris.

David.

Dear David! Now your Q is clear. As mentioned by others SRT is one (excellent) option, but along with very low Ls (< 0,02 kg BOD5/kg MLVSS). If this cannot be reached reconstruction to MBR is another option to control excessive sludge quantities. All other goes then for this already produced ES to reduce in further steps. Is this also of concern for you?

Hi Boris:

The question is primarily seeking the most practical and inexpensive methods employed by operators during secondary treatment - not tertiary (i.e., A/D or ATAD, etc). I believe your answer along with others indicates extended SRT is the best option if possible (I have looked at this myself on a lab scale and found the same). If influent loading is too high then employ MBR. As a microbiologist I have looked at many ideas (of others) including ozonation, chlorination, and various process configurations for exploiting metabolic pathways for reductions in the amount of sludge to be wasted (thereby reducing the cost of tertiary treatment - a very large expense) - BUT it would be good to know which have had the most success as many of the ideas in the scientific literature propose very large reductions in sludge volume (eg, I believe Perez-Elvira et al 2006 provide a review of proposed methods for "Sludge minimisation technologies" also Wei et al 2003 in "Minimization of excess sludge production for biological wastewater treatment" Water Research 37(18). Of course, any process must be flexible. That is, there may be times when it is more practical and less costly to operate at longer SRTs rather than employing an MBR or the OSA (Cannibal??) sidestream process. In the end the cost of treating and stabilising waste excess sludge is usually reported as the greatest expense for an operator. So any way to reduce this cost without causing an equivalent increase in secondary treatment expense is of great interest.

Thanks Boris.

Hi David: Your A opens a new Q to me ... i.e. What exactly do you mean with the term "tertiary treatment". As far as I know, primary, secondary and tertiary treatment along with polishing are used for (waste) water treatment - i.e. water stream, or line of water. Whereas the sludge is treated separately through sludge stream, or line of sludge. Thus, terms as primary, secondary and tertiary are meant for wastewater treatment ... Do we speak the same (professional) language?

And - Yes, David! Your analysis is correct: as long as the energy needed for prolonged aeration is cheaper than anaerobic sludge processing (although we gain energy here!) - go for extended aeration ... There is ongoing research on thermophilic sludge digestion at temperatures even above pasteurization level - but I guess you are not willing to go into fairly complex techniques ... so old good oxidation ditches from the Netherlands may still not go into the history ...

Hi Boris: Your question regarding tertiary treatment as I would define is the process/processes that are employed for the stabilisation of WAS (waste activated sludge). I.e., in basic terms - after clarification (CGS or DAF) you will have an amount of sludge that will be returned to some point in the 'main' secondary treatment plant (your RAS) depending on configuration (Biodenipho, UCT, Johannesberg, good old Pasveer aeration ditches) as in any AS process. 'Polishing' as I have heard the termed used by operators (it is has not been a term commonly used in microbial articles) is related to final slduge phosphorus concentration (forgive my memory if incorrect). IF you haver good sludge separation during CGS/DAF (and no upstream bulking issues) then your liquor is 'wasted' to an external receiving body (stream or UV sterilisation ponds). As far as 'complex' techniques go - again what often sounds simple in theory (i.e AD) is from what I have learnt from operators often a borderline process with multiple difficulties as can be ATAD and other microbial based thermal digestion processes. Not sure if this helps your 'Q'

Cheers Boris.

Another process that can be incorporated into the AS system that reduces excess sludge production is the side-stream sludge hydrolysis.

The main purpose of the process is to produce volatile acids for the Bio-P process, but additionally it can reduce excess sludge production.

It uses a tank which a fraction of the return sludge is passed through and is operated with a HRT of about 24 h.