The obvious answer for both color and odor is that the aeration is not sufficient... Maybe the aeration membranes (if those are used) need to be cleaned. There is a chance that some of the pores are clogged, resulting to large bubbles and worse oxygen transfer. 

Maybe it would be worth checking the quality of aerobic activated sludge. If organisms that are the componentents of this sludge are not active enough this could lead to the problems with flocculation of suspended solids and reduction of COD and BOD.

It sounds like you are getting some anaerobic areas in the aeration tank.  Perhaps solids are settling in certain areas of the tank bottom.  This will tend to produce sulfides which have a strong, rotten egg smell and tend to produce dark colored water.  I suggest adding more aeration or reducing the input of material to the tank.  Check the bottom of the tank if possible to determine if there are depositions of solids. 

Good answers, so measure sulphide concentration in influent and dissolved oxygen in aeration tank. 

Take corrective action as required – increase aeration and dose influent with either sodium hypochlorite or ferric chloride if DO < 1.5 mg/L or S > 1 mg/L.

The reason is that the iron sulfide is dark color. Iron sulfide is removed by reaeration but It is often precipitated.

Thank you for everyone sir!!!

I'll try as you are saying and I'll update you.....

There must be too much sludge settling thus some anaerobic regions exist as aeration is not uniform in the aeration tank.

It sounds there are some anaerobic regions in the reactor. Maybe due to insufficient aeration.

Hello Udayanga,

In aerobic treatment plants, aeration and maintaining proper dissolved oxygen concentrations are the key factors for the treatment. if you can provide with the COD and BOD levels of the inlet water and the HRT maintaining/aeration time it will be very much easier to suggest the possible reason. also, dissolved oxygen (DO) in aeration tank. You mentioned that the plant is for the catering service. Generally that will be having high COD and BOD. it requires long aeration time. If the oil trap is working efficiently, the major source for COD will be carbohydrate based that can work efficiently with aerobic treatment. besides I also agree with the other comments suggesting about the anaerobic regions prevailing in the system. I worked on activated sludge process (ASP) treating effluents of food manufacturing company. according to that experience it should work well. if you provide details of input parameters such as COD, BOD, MLSS, DO, HRT, type of aerators (diffused/surface), etc., it will be easier to suggest. 

Aeration and mixing are not optimised in the aeration tank, you may have some dead zones (anaerobic) or a too low level of dissolved oxygen

It means you have incresed the organic load to your aeration tank as a result there is a shortage of oxygen in the aeration tant .This can be checked by measuring DO in the aeration tank & should be  about  1.5 to 2 mg/l.

A) Please check for malfunctioning of Aeration system fouled membranes etc

B) You can improve the situation by cutting down load to the aeration tank  by reducing the flow or the inlet BOD concntration or by adding more air to th aeration tank.

C) Ensure that you are not dumping some chemical like food preservative,detergent, oil or off spec product in the waste water system  as this can increase the ETP load.

It  will take about 4 to 8 hours to restore,  if you balnce the BOD load and the oxygen supply ( 1.5 Kg O2/Kg BOD loading) subject to you have not spolied the biomass otherwise it may take 2 to 3 days to bring the normalcy.

If you need  more correct solution you  may have to share design  and operation details

Note : The black colour is due to shortage of oxygen ( less oxygen/more organic load) and will vanish once you restore the balance.Normally the healthy biomass should  be brownish  in colour  and smell like moist soil.

As Prakash mentioned, high BOD influent concentration can cause this effects.

Can you test sulfide content? High sulfide levels can produce this conditions (dark influent and odor).

Also you may have to check pH. Sludge might be not settling due to pH.

It seems that your WWTP is working with shock loadings or received a contaminant to which microorganisms were not adapted.

The black colour is likely from sulphide-metal deposits.

Is the odor sulfurous-rotten egg? If yes, then this is consistent with the other explanations concerning formation of metal-sulfides.  The odor would be a combination of hydrogen sulfide and other volatile organic sulfur compounds, likely methylated sulfur oligomers.

First of all, the black appearance of wastewater indicates that sludge is very light and bacteria and microorganisms collected in sludge got died and decayed. The reason is may be that aeration is not sufficient and/or the raw water is much higher than design parameter.

The appearance of odor smell from aeration indicates that bacteria are anaerobic rather than aerobic. By the way, the smell of normal condition of aeration tank is very nice and acceptable which almost looks like the smell of agricultural land (clay)

What I want to do:

1- Measure the DO (dissolved oxygen) in the aeration tank, and I am sure it is less than 1.0 mg/l.

2- Test the incoming wastewater (influent); BOD, COD, ammonia.

If the BOD, ammonia are much higher the design parameter, so it is required contractually to inclrease air blower and add more treating tank.

If BOD and ammonia is much lower, thus, bacterial will die due to lack of food for bacteria. Thus, add more external food for bacteria such as diluted sugar, acetic acid, methanol (take care for your eyes), molasses and any other richy organic material. Do not forget to bring external activated sludge from any other plant asa source of a lived bacterial since it looks like you bacteria is almost died. First add very much amount of the food, and do not foret to add sludge as described with keeping aeration ON continuously. Then, wait until the color of aeration tank becomes brown almost looks like chocolate.

Good luck.

The fruits and vegetables have enzymes (polyphenoloxidase. In the presence of oxygen they convert natural phenols into polyhydriquinones that are black and settledown.

Black color(presence of sulphide) is due to that the sludge in the aeration tank is turning anaerobic since there is no sufficient aeration in the tank. to avoid this we can increase the rate of aeration in the tank. so that  there is chance of reducing black colour and also odour.

Hi

Ithink you must be measure DO concentration and control the aerobic condition in aeraiion tank .increasing the aeration rate may be necessary

I agree with most comments above discussed. The low DO conditions inducing sulfphide formation and further precipitation of this reduced sulphur compound might explaing this dark colour in your overloaded tank. The presence of hydrogen sulphide also promotes the devolpment of filamentous bacteria.. which correlated with the bad settling characteristicis of the problem. There is plenty of literature in this issue (see papers from Barbosa and Stuetz in activated sludge diffusion systems.

Thank you for all!!! Problem was as you all mentioned, low level of DO. After decreasing influent rate and increasing aeration time, problem was solved.

when sludge starts to turn into dark brown, it is the sign/indication of lack of oxygen which means it may be already going into anoxic-anaerobic state

I am facing the same problem too about color. I will try all of the suggestions above. However one thing is the DO level in my aeration tank was well in range between 1.5 and 2.5. Also the settling is good. But the color is blackish. Can someone tell me how to check the activity of aeration tank if the treatment in aeration tank is working?

Low level of oxygen inducing sulfide production and further heavy metal precipitation is likely the reason behind the formation of black colour

@Soumya. If you have sulfides in the influent which are precipated by iron (dosed for P removal) it will give the black color while the treatment is functioning. The color of the sludge is not a problem by itself. You should be concerned with having low concentrations of BOD and ammonium in the effluent.

HI 

 I am facing a similar problem as Ms Soumya. The Aeration tank succeeding an digestor  turns black and the COD reduction efficiency is reduced when the feed is increased. the D.O is 1.5- 1.9 in the aeration tank. If its only the rise in the organic load , the DO should have dropped . But it shows a healthy value.

I have problem like this, oxygen not enough in my aeration tank and sludge turn to black, pH >6 . If I adding more air and decrease substrate,  it will turn to be normal right?

Dear all. 

I have just started a treatment system too. But my issue is sludge wash out. I am using OECD guidelines to simulate the treatment in the lab. What can I do to prevent this from happening. Every morning when I get to the lab I find all the sludge floating in the settling tanks (clarifiers). What can I do to prevent this? 

 An increase in recirculation rate and/ or increase of sludge removal can help , the measurement of sv30  , SVI  and MLSS in the reactor can help in determining what is the case. 

I agree with most comments about a DO level too low and about the answers of Raul Munoz and Henrik Rasmus Andersen about the impact of the presence of hydrogen sulphide and sulfide production.

Check your in-fluent flow, dissolve oxygen and industrial waste in your plant.

Hi I am facing same issue here. Mine is SBR type. DO is 5-6 mg/L. Last time I increased it and it was success to remove ammoniacal nitrogen. Now I have facing issue where is there is dark spot in the tank. When I use external blower and blow at some area, there is dark spot has come out (initially the color was brown). So I decided to remove the black sludge by manually selecting the black sludge spot by using flexible hose and suck it from top. This has been done while monitor the color at the hose end. After finished, I load new batch of effluent. After 24 hrs, when doing SV30 I found that the supernatant water become yellow transparent water. Normally is clear and transparent water. Then the ammoniacal nitrogen is not reducing. Usually when effluent load (30 mg/l next day become less than 10). But now onyly reduce to 24 mg/L. My ORP reading 190, pH is 7.56, tank capacity is 15m3 with 10 unit of aerator. Any idea how to remove black sludge, and why after i remove the black sludge the operation become upset.

I have a question, in the aeration tank the activated sludge decreases considerably, what is the cause? and how to increase the concentration of biomass?

Janitha Udayanga already the experts have nailed it and i do agree with the comments of Prakash Pimparkar and Amir Fakhrul Islam Abdullah

sounds really interesting when it is with SBBR type

Agree with @Henrik Rasmus Andersen. It is sulfides, especially iron sulfide, ammonia in combination with organics present in water.

Khadija Zaki Reasons could be F:M ratio is low, mixing and aeration in the aeration tank arenot sufficient. Check the F:M ratio regularly for proper treatment lower F:M would result in lesser biomass concentration. As microorganism can't survive properly in this situation. Try to increase mixed liqour suspended solids so that lesser biomass would be needed to treat ww. And more biomass will be available for proper clarification of water.

Sadia Afrin Thank you very much

Sadia Afrin I have an equal F/M ratio of 0.2 to increase it how can I calculate the amount of food in the form of BOD to add to the reactor

Thank you