The fact that some organics are resistant to biological degradation and do not induce oxygen depletion doesn't mean they are not dangerous for aquatic life. This especially worth for compounds created from men activities that introduce in the consolidated life cycles perturbations whose effects are difficult or impossible to anticipate.

We have the same question here in Denmark: We have a national COD limit of 75 ppm. Due to this limit we have a large municipal WWTP with significant input of industrial wastewater from some large biotech industrie. This WWTP has to apply ozonation to reduce COD from 125 ppm. Since the COD is not toxic in any assays and is biologically stable it is hard to see the environmental good that is gained from the large electric energy consumption that is used to make the ozone. One could say that we are treating a parameter rather than an environmental problem.

This is really interesting. It sounds like you're saying that this wastewater will have a chemical oxygen demand as determined by standard analytical chemistry methods but that the same wastewater when put in an actual receiving water does not deplete any dissolved oxygen. I suppose that is because the standard methods for COD are much better at degrading organics than any process in the aquatic environment. Thus, the problem is that the COD test is not really appropriate for the receiving waters in question because of these recalcitrant organics. Am I understanding this correctly?

Presence of organics in aquatic environment has primarily two impacts: (a) dissolved oxygen (DO) depletion, and (b) other negative impacts like chemical toxicity. In case of recalcitrant organics, DO depletion may not be a major problem as mentioned by the questioner. But their other negative impacts like toxicity to aquatic ecosystem can't be discarded. If such recalcitrant organics do not pose significant negative impacts (e.g., chemical toxicity) to aquatic ecosystem, then wastewaters containing the organics may be considered being safe to discharge without treatments. However, we still need to look at this point from other aspects for safety reason.

Thinking about what Rabindra is saying, is it possible that while the compounds themselves are recalcitrant that other byproducts that could form from them are toxic? Perhaps that is the reason for the precaution required in the regulation to lower COD generally. I don't think this is related to disinfection byproducts as that is a problem with recalcitrant humic materials in drinking water treatment rather than in wastewater. But is it possible that products that are toxic could form in the environment through natural processes because of the presence of the organics in question (e.g., minimal chemical transformation to create low concentration but high toxic potential substances, recalcitrant organics serving to catalyze reaction products that are toxic)? I suppose much of the answer to these questions relates to the particular identity of the recalcitrant organics.

@Nathan. You are correct. This water has a COD above 75 ppm, but BOD less than 10 ppm in standard tests.

This is not so uncommon in WWTP with part of the waste water coming from from large biotech industries, but it also occur in more traditional pure industrial waste water e.g. from paper mills. I have worked on 3 of these treatment cases and the question is always if the energy consumption is really justified by the environmental improvements gained by the treatment.

Dear Jan, Henrik & co ...!

I understand the question more in the way of environmental and finally human toxicity and adverse hormone reactions. For addressing this matter the remaining COD from the WWTP is relatively irrelevant. So, other criteria have to be set up - I propose focused toxicity tests e.g. comet test on human cells.

It is true that O3 treatment is very expensive - in this way other AOP (Advanced Oxidation Processes) have to be checked, too. E.g. peroxide H2O2, ultraviolet UV, Fenton, ultrasound US, or better ultrasound cavitation USC, hydrodynamic cavitation HDC, and their combinations. Other advanced separation processes are also worth of consideration, e.g. membrane filtration, electrochemical separation (electrophoresis), electrocoagulation, etc.

When water is well pretreated, active carbon may be cheap and efficient solution, too.

I personally see this branch of water treatment will make a big development in the future.

I would also prefer to have effluent limits only for specific chemicals and parameters which are linked to a harmful effect. The law specifies a limit for COD now, but we can try to influence this by interacting with the politics of our environmental authorities and parliaments. I think the winning argument could be LCA comparisions of the environmental effects of producing the energy used in the treatments compared to the environmental impact of the high COD.

Dear all,

Thanks for your valuable responses and comments. Sorry for the late reply.

I would like to contribute to this debate also with my views and comments.

I agree with Luca and Rabindra. It is estimated that approximately 1,000 new chemicals finds each year their way into the environment as the end result of marketing, use, and disposal. There are many new synthetic pollutant released yearly into wastewater. Degradation of many xenobiotics/micropollutants depends significantly on solid and hydraulic resistance times, biochemical environment. Many of them are very slowly degradable in municipal wastewater treatment plants and they behave as ´recalcitrant´. Micropolutants or emerging contaminants, e. g. pharmaceuticals, personal care products, pesticides, surfactants, industrial chemicals, etc. are very dangerous at very low concentrations and many of them are removed in municipal wastewater treatment plant with very low efficiencies. On the other hand, we have limited knowledge of the environmental transformation, fate or impacts of these compounds.

Rabinda have mentioned safety reasons… I consider safety reasons and many uncertainties regarding behaviour and fate of many pollutants including long term effects as main reasons for the policy and practice of conservative effluent COD standards

Dear Henrik,

Municipal wastewater treatment plants (WWTP) effluents have traditionally been regulated based on specified level of technology or end-of-pipe effluent quality criteria. It is questionable if this practice always guarantees environmental protection as well as compliance with all environmental regulations. On the other hand, according to my knowledge and experience involvement of tertiary ozonation in municipal WWTP is not common. I would like to ask you:

1, What is the contribution of industrial organic load to the total organic load of the WWTP?

2, Did you perform also measurements of some specific synthetic substances at the effluent from the WWTP?

3, Did you perform also some long-term tests?

Regarding your third comment - I guess that basic objective of your WWTP is removal of organics, maybe also nutrients and of course appropriate removal of priority and relevant pollutant if they are contained in the wastewater. Thus, one of the main control/controlled variables are COD/BOD, particularly COD as this methods gives information in more ore les 2.5 to 3 hours. So, also in the case you will have reliable procedure for control of specific chemicals and parameters COD/BOD values will be monitored.

Dear Nathan, I agree with you, that the standard method for COD determination is much better at degrading organics than any process in the aquatic environment. However, municipal effluents are considered to be principal point-source of xenobiotics/micropollutants (MP) including chemicals that may alter endocrine function, thereby adversely affecting reproduction or development in animals. Some of them are removed at least to some extent due to biological degradation process, i.g. due to higher solid retention time (SRT), but mainly by abiotic processes, i.g. adsorption on activated sludge. Unfortunately, many of xenobiotics/MP pass trough WWTPs and are discharged into receiving water. There is new environment, processes, interactions… For example, sediment as small ´reactors´ with dominant anaerobic environment. Many chemicals in municipal effluents are hydrophobic and may tend to adsorb to particles in the effluent or sediments in the receiving environment, rather than remain in the water phase. On the other hand, discharge f nonylphenol poly ethoxylates degrade under anaerobic conditions to 4-nonphenol which is a more toxic, stabile/ persistent/material with estrogenic effects.

So, despite considerable investment in treatment systems, acute and chronic toxicity remains a concern in many sites receiving municipal effluents.

You are right also regarding identity of the recalcitrant organics. There are different interpretations of recalcitrant organics. Strictly, recalcitrant organics should not undergo any biological degradation neither under anaerobic nor under anaerobic conditions. Slowly/very slowly degradable organics are usually called as resistant. However, many times, and this is also the case of my question the terms resistant and recalcitrant are not strictly differentiated.

Dear Boris,

You also named the problem correctly. However, I support the measures for decrease of COD content in WWTP effluents including incentive policy on fees for wastewater discharge in order to reduce potential risks. I agree with toxicity tests. Unfortunately, this is not my field. I will appreciate your comment on what kind of test would you recommend with regard to prevention both human health as well as aquatic health.

All of mentioned treatment processes have a potential, advantages and drawbacks. Main advantage of most chemical degradation processes is that there will be no sludge compared with separation processes.

Currently, it seems to be most promising in terms of the removal micropollutants ozonation with the following sand filtration, in terms of removal efficiency and the elimination of the term toxic effects on the aquatic environment. Currently, for the removal micropollutants seems to be most promising ozonation with the following sand filtration or activated carbon, with regard to treatment efficiency and the elimination of toxic effects on the aquatic environment. I recommend the paper:

http://pubs.acs.org/doi/abs/10.1021/es9014629

http://www.ncbi.nlm.nih.gov/pubmed/19863988

You are right, it is relatively expensive process. Main reasons are the low ozone concentration in air or oxygen (approximately 4 to 8% respectively) which is coupled with the very low (5 to 10%) energy efficiency. But, this is the challenge for producers of ozone generator as well as development of reactors with higher efficiency of ozone utilisation.

Regarding activated carbon, there are not doubts on sorption affinity and capacity. But in the case of real wastewater, particularly industrial there can be problem. We tried to use it for removal of already mentioned nonylphenol. We observed very low removal of these pollutants with activated carbon. Higher affinity was observed when using zeolite or excess activated sludge which was not loaded with this compound before.

I also personally think that this branch of water/wastewater treatment will make a big development in the future.

Dear Jan ... and others!

Sorry for not answering promptly .... Er, nobody else did answer??? for 4 weeks? Seems that we have said everything !? ...

Right now we are in the middle of a study where we tackle different wastewater with the before mentioned AOP - either with single or coupled processes ... And our results are encouraging, i.e. the right combination of the AOP may reduce a great number of before recalcitrant chemicals. May I advise you to see our last paper: https://www.researchgate.net/publication/233858625_REMOVAL_OF_PHARMACEUTICALS_FROM_WASTEWATER_BY_BIOLOGICAL_PROCESSES_HYDRODYNAMIC_CAVITATION_AND_UV_TREATMENT

Hopefully, our research will give us even better results in the near future. Stay in contact!

Article Removal of pharmaceuticals from wastewater by biological pro...

Dear Boris,

thanks for your answer. I read the abstract of your last paper. Very nice results, indeed... Particularly, the efficiencies observed by using combined processes are excellent. However, the results of biological treatment processes are also very promising. Unfortunately I do not have access to full text. Will you be so kind and provide information about solid retention time maintained during operation suspended activated sludge?

@Jan: ??? Ups?! I thought that I uploaded the full text that was submitted for publishing ... Something went wrong ?!? If you disclose your e-mail. I can send you ...

Thanks Boris, I will appreciate it

My email: [email protected]

You are right Grant. The question is how to remove effectively these micropollutants (MP) and protect water environment including drinking water resources? There are many processes under investigation.

One of the cheapest options is biological treatment. It is well known that some xenobiotics can be removed by biological treatment processes after acclimation to higher solid retention time (SRT). The contribution of abiotic removal, i.e. biosorption can be also significant. Both, biotic and abiotic removals are significantly influenced by SRT. This is the reason I asked Boris about the SRT value they maintained during operation activated sludge process with suspended biomass.

Unfortunately, biological treatment processes are not efficient sufficiently for removal of all MP. So, many other separation and degradation processes and their combinations, i.g. the combinations of biological processes with hydrodynamic cavitation or OV treatment, presented by Boris, are intensively investigated.

As I already mentioned among very prospective degradation processes belongs ozonation in combination with sand filtration or activated carbon. Other promising option seem to be membrane separation processes.

Dear Jan,

Thanks for your comments and I agree that ozonation of municipial wastewater effluents is not common. I have only done a small part of the work on the investigation and engineering of the solutions of the treatment case I was writing about. Therefore my answer to your questions is not more detailed than my general knowledge of the plant:

1) The fraction of industrial WW to the plant has changed a bit over the 10 year the case has been considered, but it is about 50% of the volume treated.

2) As the industrial wastewater is from biotech industries I don’t believe there is any monitoring of specific chemicals except from the chemicals required for normal municipial effluents according to the Danish regulations and monitoring programs which includes something from the EU Water framework Directive.

3) The ozonation has been running full scale for at least 5 years, but I would not call that a long-term test.

The municipial effluent has the main control parameters to have BOD (

Dear Jan,

one thing I would like to mention is the sheer number of these micropollutants in municipal waste waters. although the individual concentration can be as low as few ng L-1, I measured some of those contaminants at concentrations of over 66 µg L-1. and the complex interactions between those contaminants, may they be stimulants, antibiotics, personal care products etc onto aquatic organisms and in the end onto humans is not fully understood.

One other thing I would like to mention is that even with a very strong AOP like photo-Fenton, the degradation of some of these contaminants is not complete in a reasonable time.

There are some investigations going on at the moment if it is economical to couple AOPs and ozonation with sandfiltration techniques.

Thanks Nikolaus for your comment. There is no doubt that these processes are expensive. But, which are alternative cheapper processes for micropollutants removal?

For Henrik Andersen:

Thanks Henrik, so the COD limit of 75 ppm is the only reason for implementation of ozonation process as tertiary treatment?

For Jan: Yes.

AOPs and ozonation are extremely expensive processes compared to the other water treatment processes we use on large sewage treatment plants.

Therefore extreme process optimization and taking full advantage of biological treatment before and after the treatment is necessary.

The industry that sell or use AOP are usually very interested in university co-operation so there are many thesis projects that can be made on analyzing treatment cases and making laboratory scale test treatments.

Well Jan, as others here already mentioned, these processes are extremely expensive. one way to lower costs may be the use of natural sunlight (compared to UV lamps) the extreme optimization of the involved processes and the coupling of biological and oxidation processes. another possibility is the application of membranes to "filter" contaminants out of the water, and the subsequent treatment of the retentate with AOPs or ozone.

the field is vast.. :-)

It sounds very well. It seems that also more expensive treatment processes should be implemented in WWTP, at least in specific cases. On the other hand, you never obtain such valuable experimental data in laboratory... and this approach is very convenient also for companies..a little bit I envy... Are you involved in this project? Can you recommend some paper dealing with operation of that WWTP, particularly with operation of ozonation process?

This comment above is for Henrik. Sorry Nikolaus, I did not register your answer...

I will respond to your comment later/tommorow ...

FOR Nikolaus:

Nikolaus, I agree with combination of chemical /AOP and biological processes...

To be honest with you, I am not experienced regarding the potential of solar energy for improvement of effectiveness of AOP in practice.

You have mentioned also membrane separation and subsequent treatment of retentate by AOPs. Do you consider the application of NF/RO in combination with AOPs to be cheaper in comparison to combination of AOP with biological treatment? I also would appreciate some references on papers dealing with costs evaluation of such processes if you have.

FOR Jan: There isn't anything published about the experience gained from the long term running of the full scale project. Something has been presented in national conferences in Denmark and I have talked with several of the of the people that have been responsible for the design and the running over the years. I think the message I hear from all is that they could have benefited a lot (millions of €) from running a pilot plant for a long time.

I am not continuously involved with this plant but I have access to it and a few other slightly smaller installations with the same problem that I can use for student projects and projects with companies that develop technologies. Due to the time I have to spend on other fine projects I am not doing anything with COD removal just now, but I intend to get back to it and I enjoy that it is quite easy to get funding for developing this types of treatments compared to the treatments for specific micro-pollutants (pharmaceuticals etc.) which many researchers work on.

FOR Nikolaus: The search for solar based AOPs have been on for 40+ years, but still we have not seen real plants being constructed even in the areas like Almeria and Israel that have both sun and technical capacity.

The most general problem is the availability of the sun then and there the treatment is required. You need to store water in the night and on rainy days and storage capacity for wastewater is very costly. Then most of the wastewater that needs advanced treatments is not even in sunny countries.

I have a lot of respect for the work done at Plataforma Solar de Almeria and I think the work from this group can eventually be used to create some cost effective AOP techniques for the areas that have much sun.

Henrik, thanks for response and comment. I´d like to ask you about benefits. Do you mean real benefits due to improved quality of discharged effluent or hypothetical benefits by saving energy for ozonation?

FOR Henrik,

There is a treatment plant in El Ejido, close to Almeria, which washes pesticide contaminated plastic containers for plastic recycling. The water is then treated with solar photo-Fenton combined with MBR. This plant is able to treat 1.6m³ of highly contaminated water with a collector surface of 150m², but this is just a very specialized plant, and works in batch mode. so you are right, there are no huge water treatment plants for this sort of thing. but as you say, they are working on it.

FOR Jan,

I am no expert in NF/RO treatment, and I did not make any cost evaluations on that. sorry. my research was purely on the development of an AOP for the degradation of these contaminants. but I can have a look if someone from my former department has done work on this.

Thanks Nikolaus, I will appreciate also a cost evaluation for AOPs, if available. It would be nice to have it for both, NF/RO and AOPs

Jan: I am talking about construction cost and efficiency of the ozone (and energy) used. The effluent is treated to the desired quality, but it is strongly suspected that this could could have been done at a lower investment and running cost if the system could have been developed further.

Nikolaus: I think the idea of pre-concentrating the wastewater with NF/RO before using AOP is very interesting to explore. I have heard it mentioned before, but never seen any papers that analyse the combination.

It is not easy to see how much the increased concentration of colored matrix and radical scavengers in the concentrate will increase the "dose" of AOP needed to remove the micropollutants. I guess the dose will not increase as much as the concentration factor, but the saving in AOP efficiency is countered by that NF/RO is also an expensive treatment.

Dear Henrik,

there is a publication of my former research group, investigating the combination of nanofiltration and photo-Fenton.

Article:

Combined nanofiltration and photo-Fenton treatment of water containing micropollutants

Journal:

Chemical Engineering Journal

Authors:

Miralles-Cuevas, S., Arques, A., Maldonado M.I., Sanchez-Perez, J.A., Malato S.

Article in Press, DOI: 10.1016/j.cej.2012.09.068

Dear all,

thank you very much for your present valuable comments and responses. We coincided that this branch of water/wastewater treatment would make a big development in the future. I believe we shall continue in our discussion also in the next year.

Happy, healthy and prosperous New Year 2013 to all of you!

Hi Jan,

Biorecalcitrant could include a variety organics from pharmaceutical to pesticides and herbicides, etc. Some are toxic to aquatic organisms and almost all could accumulate in aquatic species and lead to, for example, feminization of fish and other aquatic species, harm non-aquatic species preying on fish, etc. There are ample evidence in the literature in this regards and studies on the effect of persistent organics on aquatic species and birds of prey, for example, in Lake Ontario and other Great Lakes.

from the pioint of aquatic environmental protection before you discharge your water it should meat the guidlines of wastewater and in this case when you have high COD value thats mean you have both organic and inorganic compounds which may affect the whole aquatic ecosystem.

at least if you dont want to pay much and for economic value you can use cheap treatment types with low cost such biologcal degredation (aeration) to put it for long time before you dischrge it. or activated carbon synthsis from agriculture waste, to achive your COD accepted value for discharge or resue

@Nikolaus Klamerth: This week I have seen a presentation by Professor Sixtus Malato of a continuation of the work you mentioned. I agree that the work in your former research group clearly show that pre-concentration wastewater with membrane filtration before destroying the contaminants (or recalcitrant organic dissolved matter) with solar driven photo-Fenton has the potential to significantly decrease the overall treatment cost. It seems quite reasonable that the combination will eventually be comparable with direct ozonation. The way I understand the system design I think the advantage only applied to photo activated AOPs, so I cannot use membranes to achieve a more efficient process with ozonation, perozone or traditional Fenton. Do you agree with this interpretation?

@Mohamed Hassaan: Part of the discussion of the question has been on if it makes sense to treat wastewater to satisfy an effluent criteria for COD if the organic matter that that causes the high COD is not readily biodegradable (no oxygen depletion in the receiving water) nor toxic.

We have treatments to fulfill the COD criteria by Fenton oxidation and ozonation in some facilities. This causes large treatment costs and energy use. If we were to select not to regulate COD we could use this funds and energy to solve other environmental problems.