Dear Christoph,

Thank you for the long answer on the short question. Great that you found the time to do a quick re-cap of the evolution of the term DEWATS.

I agree with you that the effluent of anaerobic processes should not go into water courses, which is probably why the original DEWATS concept has been changed over time .

As I said here in Nepal often a combination of ABR and wetland is used, wetland can be either horizontal or vertical. Because there is very little flat land in Nepal it is usually quite easy to achieve this without pumps and have everything based on gravity flow. One other interesting innovation they built at the Enpho office here is a combination of septic tank and vertical flow wetland. The way they achieve intermittent flow for the wetland is by having a siphon in the second chamber of the septic tank. If the effluent level rises to a certain level the siphon flushes like a toilet. Then it takes some time again for the fluid level in the septic tank to reach that maximum level. I have feeling this is not new to you, but maybe for some other people it is .

regards

Marijn

Hi,
as most might know the DEWATS term was created by Borda. It meens Decentralized wastewater Treatement systems. The idea was described in detail in SASSE, L. DEWATS - Decentralised Wastewater Treatment in Developing Countries. Ed. BORDA (www.borda-net.org). 1998
Basically the idea of DEWATS is extremly low maintenance systems. At the time they classified (writing out of memory):
Baffled reactor, anaerobic filter, septic tank, and horizontal flow wetlands as DEWATS systems whereas
UASB, Vertical flow wetlands and very technical treatment are NOT DEWATS Systems.
They draw a very hard line between systems which are - for the authors - considered low maintenance and others.
As I´m a “verticalist” concerning wetland systems and I defend very much simplicity of treatments as well, I never felt very much comfortable with that definition. The vertical flow systems for instance got dismissed as DEWATS as they often need a pump.
I really do appreciate the publication and the idea of having simple systems and a name for simple systems, but I feel we have to be very clear where are the limitations of DEWATS as well and open minded for the solutions.
I always felt that the aspect of effluent quality achieved in originally called DEWATS systems is not as it should be, as the treatment is mostly anaerobic. I do have a strong concerns about that aspect.
I heard and saw quite a few systems which are still called DEWATS which tried to solve the effluent quality aspect, but I do not agree with the idea that they are simple… much less low maintenance. So I would not call them DEWATS. I saw a baffled reactor with 5 compartments, followed by an anaerobic filter, a pumping station and a vertical flow wetland. Where is the idea of simplicity? Is the Vertical flow Wetland now accepted? What about the pumping station?
I appreciate the DEWATS idea (not the systems which were classified at the time) but the latest developments need rethinking I guess. I know that it is always a problem, that a label is taken up (which is good) and everybody starts to build (which is good), but just claiming that the system is DEWATS (which is very bad) not sticking to the basic ideas of DEWATS.
I really do not understand what is the purpose of a baffled reactor (which is anaerobic treatment) followed by an anaerobic filter (which is an anaerobic treatment). When I have a good functioning baffled, my anaerobic filter is worthless as the combination anaerobic – anaerobic does not add value (but a LOT OF INVESTMENT COSTS). When my baffled is not working as it should than I have to check the concept and nod add another anaerobic step. I get nervous when I see the discharge of two pretreatments (Baffled followed by anaerobic filter) going into a water course. For me that is NEVER ok. There is no reduction of nutrients (as we discussed before) and –worse - there is no significant elimination of helminth eggs nor coliforms. In my regard the anaerobic effluent has to be infiltrated ALWAYS, if not, it might cause a worse situation for the water body than before where a lot of the wastewater did not reach the water body.
My absolute favorite for decentralized treatment are the French system wetlands although they are not possible to implant in many cases. The few we did are very stable and delivering a very high quality effluent. My second best option is a simplified baffled (just 3 baffles, due to cost reasons and inflow in the first chamber as well at the bottom) followed by a vertical flow wetland. We have build (and are operating) about 10 of this combination (up to 2.500 pe.)and they are functioning very stable.
This solutions are almost the half in costs in comparison the latest DEWATS systems I saw.
Sorry, a very long answer to Marijns short question but it is an issue I had in mind since a longer time to comment on. I hope it does not sound polemic, my intention is not to have a polemic discussion, just a reflexion of about a very good term which has to be developed further.
Yours
Christoph

Dear all,

I am slightly confused by some aspects of the discussion (I think this is partly a definition issue of what DEWATS is).
Frank, what type of units are you describing here as having effluent with a high NH4-N concentration? I assume septic tanks or ABRs?

Here in Nepal, DEWATS is usually seen as a combination of technologies, one pre-treatment step like an ABR followed by a wet-land (or a bio-gas based system, which only has sludge as output).
I have no figures on NH4-N loads in the effluent of the CW, but especially for vertical flow I think it would be quite low.

Kind regards

Marijn

Hi Frank,
answering your questions

Furthermore, aereation as a second step might then even increase the fish toxicity, if O2 is not added sufficiently (NO2!), or the nitrification is inhibited somehow. Is that right?

That should not be an argument not to do a secondary treatment...but yes it would be right if you are producing NO2. That is the reason NO2 is a parameter which is to be monitored especially in starting up a nitrifying step.

Can you confirm that this is a frequent (?) / well-known (?) problem in anaerobic plants?

I can NOT confirm that it is a problem for Anaerobic pretreatment that NH4 increases. Normally the NH4 values stay the same. They are only high in the effluent of digesters where the digestion of proteins sets free large amounts of ammonia.

In our case we have discharges from anaerobic digesters in our inflow to the plant.

I would definitely avoid the influent of digesters due to the mentioned problem. But just a doubt. Digester? Before a Dewats plant? Anyhow. If you remember your masters thesis nitrification is VERY VERY effective in vertical flow wetlands. So if your stream is not very very high you might try a wetland especifically for nitrification of the sludge liquor. But in needs tryout, nothing to do in scale.
Yours
Christoph

Thanks, Christoph!

we have seen NH4- effluent vales from DEWATS-like plants up to 150 mg/l! In that case you do not even need a high ph-value in order to have a remarkable Ammonia-concentration in the effluent.

Furthermore, areation as a second step might then even increase the fish toxicity, if O2 is not added sufficiently (NO2!), or the nitrification is inhibited somehow. Is that right?

Can you confirm that this is a frequent (?) / well-known (?) problem in anaerobic plants?
In our case we have discharges from anaerobic digesters in our inflow to the plant. They are adding a high input of NH4 already. Is his something that should be avoided? But where to discharge those effluents then?

Thanks
Frank

Dear all,
I would like to comment about some mentioned aspects:
a) Ammonium (NH4) is not directly the problem for fish. Ammonium (NH4)and Ammonia (NH3) are in direct correlation depending on the pH. On higher pH the Ammonium (which is directly present as Ammonium or TKN in the raw sewage) switches almost completely to Ammonia. Ammonia is fish toxic from concentrations below 1 mg/l therefore a direct problem for small water courses.
The diagram in the follwing link shows the relation.

b)the normal way of elimination of ammonium is via nitrification hich is via NO2 (Nitrite) to NO3 (Nitrate). The NO2 stage is extremely fish toxic already in very low concentrations (0,01 mg/l).

Therefore there are various reasons why ammonia in small streams is a problem, besides the oxygen consumption for nitrification.

Eutrophication is not that much of a problem as in most cases P is the limiting factor.

Yours
Christoph

Ammonium or ammonia results under anaerobic conditions. This, together with hydrogen sulfide gas, formed in anaerobic systems when the influent contains high level of sulfate ions (600 mg/l and above), gives rotten egg-like smell. Anaerobic effluents also cause oxygen sag in the receiving streams. The oxygen sag may be temporary in well-aerated stream, but in streams where there are a number of point discharges and there is less turbulence in the stream, the oxygen sag may be permanent (e.g., klongs in Bangkok). My suggestion is that, if you love anaerobic systems, have an aerobic unit as an final treatment or polishing unit.

Ammonia is toxic to fish ONLY at high ph (10 and above).

F H Mughal

Dear Frank,

I am also keen to explore this topic. We are planning to construct a DEWATS adjacent to a small urban bond which is used for washing of clothes and utensils, personal hygiene and also fishing. I'd like to add that NH4 enhances eutrophication and the prodcution of algae which might release toxins within the pond which can cause a risk for public health.

Discharge guidelines according to the Environmental Rules in India are 50 mg/l NH4 (as N) to inland surfaces waters. However state boards can lay down sticter effluent guidelines adpated to the local conditions as rivers have a way higher capacity to oxidize NH4 themselves.

Cheers

Dear all,

I am interested in the issue of NH4 in effluents from decentralised / small-scale waste water treatment plants (anaerobic / DEWATS-type).

I have the feeling that might be an underestimated problem. NH4 in higher concentration is fish toxic. The concentrated discharge into a surface water may trigger problems with downstream users (fishers) etc., let alone water supply facilities.

How should effluent standards on NH4 for small plants look like?
Are there any studies around which you are aware of?

I am looking forward to the discussion.
Thanks and best regards
Frank