Operation Problems

[Chris73]

Hi Eng's:
One of some small Sewage Treatment Plants I am responsible for, features a strange performance over the last weeks. It's a small extended airation unit for a Hotel-Lodge. The capacity is enough and it ran well for years. 3 Months ago they had to relocate the unit and for new commissioning we inoculated the plant with healthy sludge from a similiar plant. Since that it never worked well again. The sludge is black and odorous (septic smell), oxygen < 1 mg/l, pH 9.5 - the bugs are obviously dead. There was never any different influent than before (I've still sent a sample to a lab for toxicity tests). I turned the airation to full but can't really explain the happenings. Anybody has an idea?

 

[rbcoulter]

Did you check the phosphorus level?

 

[Chris73]

I have induced a P check at the lab - haven't got the results yet. Do you think that P could cause such performance? Nutrient ratio C:N 100:5:1 can affect the reproduction rate but can an unbalanced ratio kill all the aerobic bac-stocks?

 

[rbcoulter]

The phosphorus level is a routine check, may not be the problem. I just noted that your oxygen level seems pretty low. Usually when aerobic bugs die, the oxygen level goes up IF you are aerating correctly. This makes be believe that your are not supplying adequate
aeration. What is your aeration system. Diffused air or agitation? If diffused air, your aeration grid may be plugged and therefore not supplying enough oxygen.
This can happen after a shutdown because the grid plugs up with dried sludge. If you don't get enough air into the system, anaerobic takes over.

 

[BobPE]

I would kinda agree with rbcoulter, have you done anything during the system relocation to affect the blower system? See if you can get a reading on air volume being moved and compare that to the theoritical calcs for that type of system.

Bob

 

[Chris73]

Just got the results from the lab - unfortunately they could not carry out a toxicity test in lack of sufficient sample liquid. The BOD is unusual high 588; P is ok with 8 mg/L, pH = 7.2 ok too. Due to the bad operation the effluent BOD is still 277. Got the lab to check what percentage of BOD comes out of solids and dissolved ingredients. Next time I will let them check COD as well. Now I am wondering what on earth can cause such a high BOD in a Lodge, which comprises of maybe 70 people? And then all of a sudden, because before the move BOD was in the range of 200. Aeration seemed to work alright - will check the size of the bubbles closer next time but I did not have the feeling that the diffuser membrans were plugged (hard to say from on top anyway). Well, I hope that brings some more ideas up from you. By the way, thanks a lot for your support!

 

[rbcoulter]

It was always difficult for me to look at the top and determine if aeration was adequate. Looks are deceiving. Do you have a pressure gauge on your blower discharge?

 

[BobPE]

Chris:

BOD can be really crazy in a seasonal environment like that. Occupant loading, cleaning loading, and food prep loading all have impacts and play a role at various times. Like rbcoulter is saying and I am tending to agree, I think you air system is not working too well. On seasonal designs i use a flow meter for air simply because the operators are usually not up on the day to day operation of the system and I wrtite operating instructions for the o and m manual based upon flows during various times of the year. Look for a pressure gauge and see if you can use the blower curves to determine the air output. Dont look at the bubbles it wont tell you a thing in these smaller systems.

What type of system is this? Is it a package system and if so who made it? Maybe that will help us some.

Bob

 

[wwoper]

Do you have any subsequent influent/effluent BOD data and your process dissolved oxygen and pH data to tell you what is coming in, how well its being treated, and whether or not you're adding enough oxygen and maintaining good process pH? If you do, what is data? Assume this is some sort of package plant? What is design flow? actual flow? and is the flow through the plant itself constant? What is the MLSS, and is it dark and scummy (old sludge)?

 

[Chris73]

wwoper:
we inspect the plant only once a month - so I do not have enough data to provide monotoring. These are the measured data again:

DO (dis. ox) aeration tank = < 1 mg/l
DO (dis. ox) clarifier ~ 1.5 mg/l
pH aeration tank = 9.5
pH clarifier = 9.5
BOD influent = 588 mg/l
BOD effluent = 277 mg/l
TSS influent = 192 mg/l
TSS effluent = 74 mg/l
P influent = 8.5 mg/l
Settled solids mixed liquor = 100 ml/l

Yes, it is a package plant and the design max flow is ~ 5.5 m^3/day. The actual plant capacity is somewhat lower. The sludge is black and smells septic. The flow graph may vary as usual for a small plant for lodge/restaurant business.