smelly manhole

[RDK]

A client has a manhole just outside their main public entrance to their building. The property is completely theirs (no municipal government involved, it’s a campus like setting)

This manhole is the receiving manhole for a lift station. The 6” force main discharges into this manhole where it again becomes a gravity flow to the treatment plant.

As you can imaging there are some days where, shall we say, the purpose of the manhole is olfactory apparent. Last summer the problem was very obvious on the day when a large potential client was on site.

The problem has become more acute in the past couple of years as the flows into the force main have dropped off due to a change in usage of the area served. The sewage in the lift stations wet well sometimes gets a little septic. Also when the workers wash down the wet well and stir up any accumulated sludge the problem is worse.

Some operational changes have been made, like wash downs outside of normal work hours, but the problem still occurs.

Needless to say this is not a desirable condition. I am searching for a low cost solution.

My ideas so far:

Reroute the force main Costly but effective. Lots of paved areas and roads around the area.

Seal the manhole. This would then run the risk of filling the top of the manhole with lighter than air gases, which is not a good thing.

Vent the manhole. I thought of a fan mounted in the throat of the manhole blowing downwards. This would force the air through the sewer pipe (which is never full) to the next manhole where it could dissipate in a more acceptable location.

A combination of sealing and venting where a 100 mm pipe runs from the manhole to a location where the fumes could be exhausted without disruption.

Shortening lift station pump cycle times by adjusting the floats or reducing the volume of the wet well. (Its oversized for current loading but this may change later. The size reduction would have to be removable)

Any ideas or comments?




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[BobPE]

RDK:

Thats a common problem where as you say, things change in the drainage basin served by the pump station. The diurnal flow from the station is not enough to change the volume in the force main and it is the force main that is going septic. When flow from the main does reach the manhole, it too is septic and the flow from the pump stating that puched the flow to the manhole sits in the force main to become septic and repeat the cycle.

I used hydrogen peroxide as a cheap source of oxygen that you inject into the force main while the pumps pump. I have done cost comparisons on other wonder chemicals that vendors sell that do the same thing and always come back to good old hydrogen peroxide.

When the usage of the pumps station permits it to operate at its design capacity, the hydrogen peroxide equipment can be removed. It is basically a storage tank sized for your needs and a chemical feed pump and some basic controls.

Let us know what you end up doing.

BobPE

 

[RDK]

I imagine that this could be by a venture on the pump discharge or a separate feed pump switched by relay to the main pumps.

What ratio of hydrogen peroxide to sewage are we talking about?



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[sc]

I would tend to run with the vent and sealing idea, as it is cheap and does not require much in the way of ongoing maintenance or addition of chemicals.

Regards

sc

 

[ema]

RDK:

Injection of hydrogen peroxide or pure oxygen is a good idea. Plus installation of frequency converters for the pumps could help to adjust pumping capacity and there will not be any need for removing them. Sealing and venting the manhole using ex-proof design fans could also help to exhaust the odor causing fumes.

Regards

ema

 

[KRSServices]

Rick,

In the wet well, bio-enymes (acti-zyme, etc.) can be added to the effluent. This will mitigate some odour and will reduce grease/scale buildups.

A matting can be fitted in the manhole, similar to a frost lid, which are specifically designed to reduce the odours in the manhole. I'm not sure of the manufacturer but I do know that they require regular maintenance. I used the enzymes in three wet wells, in situations you have described. In two of them it worked well due to the lower volumes of flow. With high flows, the enzymes do not have enough contact time in the wet well.

KRS Services

http://www.krs-services.com

 

[BobPE]

RDK:

Not knowing much about the volumes involved with your system, I would venture a guess of 1 to 2 mg/l of 50% or a gallon or so a day of diluted H2O2, which isn't much. Who's manhole is it? Your clients? If you seal it and power vent, you will draw H2S from other parts of the collection system. If you seal it and don't vent it, the H2S will buid up, in either event, once you do either of those designs, plan for the manhole not to be in service for very long since it will be destroyed by Sulphuric Acid and will become a structural in addition to a smelly problem.

BobPE

 

[KRSServices]

RDK, as BobPE has already noted, hydrogen peroxide works very well. The injection equipment can get a little pricey, but in the end, reduction of the odours will be the same. Regarding the enzymes, we were operating a small STP, and utilized them with daily applications into the lagoons to reduce the odours for residents living "upwind".

KRS Services

http://www.krs-services.com

 

[sc]

RDK

Grease would be reasonable for a short term seal, however, I would suggest that a longer term solution be found.

This may mean replacing the existing cover with a purpose build sealed cover or having a thin (but dense) rubber ring installed to edge of the existing lid if possible.

regards
sc

 

[jmerced]

An enexpensive solution for keeping odor in and water out is the Lockdown-Lockdry manhole security lids that you can find here:

http://www.lockdown-lockdry.com/

It's been my experience that once you have an odor problem with a manhole, it can be very expensive to remedy via removing the source of the odor. The least expensive way to go (which I gather from your request for info is a concern)would be to keep odor from escaping above ground.

 

[af7k]

With respect to flow control, I designed a lift-station with PLCs with variable levels for pumping. During the night, we lowered the start level so that it maintained a shorter residence time. That took care of H2S.

We had problems with normal sewage odors and ended up sealing the station.

Some use a venting solution where the venting is into the ground into a buried rock filter much like a septic tank leach line with an earthen cover.

More expensive solutions include venting through filters such as peat moss (commercial design, not a do-it-yourself design). We had little success with absorbent materials or various odor controlling agents.

 

[semo]

Another chemical I have used with some success has been chlorine. A lot depends on the detention time in the forcemain. You can kill the bacteria or add oxygen keeping them in an aerobic condition; but, if the detention time is long enough they will convert back to an anaerobic condition and break down the sulfates releasing hydrogen sulfide.

Another method I have heard about; but, yet to try is the addition of magnesium hydroxide. It raises the pH such that the less of dissolved sulfides form hydrogen sulfide and remain in solution as bisulfides. This is also said to help with sludge bulking at the treatment plant.

 

[91markus]

I'll second the notion of sealing and running a vent stack somewhere a few feet away from the problem manhole.

Venting and using vent stacks is almost exclusively done on industrial sewers where H2S levels are high enough to be aestetically unpleasant but not high enough where H2S levels are dangerous(which would describe most municipal H2S levels/systems).

I would recommend coating the discharge MH however, as eventually the H2S will corrode the concrete.

 

[ectropy]

At my last job, we had a manhole, which received wastewater from a long forcemain and an inverted syphon. The lid was near a residential area, and the odor was evident most days. We used an inside drop structure for the force main, with a tee on the top end of the drop, and an open-ended 12" PVC extension on the top part of the tee. The sewage was then directed to the outlet of the manhole. This created a self-oxidizing condition in the force main effluent, and also sucked any resulting odors down the gravity main and to the pump station. I found the solution in an old (1950s) wastewater design book, which I have since been unable to locate. It worked well. Any input on where I could find documentation for this method would be appreciated.

 

[Bowne]

Ectroy -

If I understand your approach to the problem, basically you used the interior drop assembly as a means of piping the sewerage flows directly from the force main connection to the manhole over to the outlet pipe leaving the structure. At the connection of the drop assembly to the inlet pipe, the top of the tee was left open to allow air to be drawn into the drop with the falling water. Fresh air, which was available through the vent holes of the manhole cover, was drawn into the structure and down the drop assembly, therefore eliminating the release of odors from force main effluent. Please confirm.

 

[HB2U]

Start with an inexpensive experiment first.

Just fit the lid with an insert. The type that's used to prevent rainwater from entering the system.

If you solve the problem for a couple of dollars, you'll look like a stud.

If it doesn't work, at least they know you tried the simplest method first - you didn;t spend a lot of moneu to do it - and then they'll be receptive to a more aggressive solution.


Good luck.

HB

 

[ectropy]

Reply to Bowne,
Yes, you understand the method correctly.