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| |  Posted 1/25/2008 3:25:20 PM | |
| Junior Member
 
Group: Forum Members
Last Login: 4/23/2008 4:07:25 PM
Posts: 14, Visits: 44 |
| | My company operates several small, on-site wastewater treatment plants. By "small" I mean 5,000-25,000 gpd. Sludge is wasted to sludge holding tanks. The tanks are buried septic tanks. The sludge is pumped periodically by septic haulers and taken to large municipal wastewater treatment plants. The frequency of the pumping ranges from as often as once per week to as infrequently as once per month. The waste sludge is typically around 1-2 percent solids, but sometimes, as in the case of a plant that we just started running, the sludge is more like 0.5 percent solids. Question: Has anyone had a good experience using polymers, metal salts, or other agents to easily and economically thicken the sludge to closer to 2 percent? If so, how was the product mixed with the sludge? I'm thinking that the ideal solution for us is to add a cup or so of some product each time a few thousand gallons of RAS is wasted, and to mix it with a submerged pump. But I don't know what product to try. Absent feedback, I'll start ringing up vendors. Would very much appreciate learning from others' experiences. Thanks.. |
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| | | Posted 1/25/2008 11:54:08 PM | |
| Supreme Being
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Last Login: Yesterday @ 5:20:14 PM
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| | I would suggest getting a couple of gallons of WAS from the plant and doing some jar tests. I don't think any one will be able to give you any better guidance as these things tend to be plant and application specific. I would start off with what ever you have or can get hold of easily. 2 percent solids might be possible but it would be certainly easier to achieve if you had a special thickener rather than just a septic tank. Regards TerryF |
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| |  Posted 2/1/2008 7:52:49 AM | |
| Junior Member
Group: Forum Members
Last Login: 2/29/2008 3:07:09 AM
Posts: 18, Visits: 16 |
| | I used to help run package extended aeration plants like this. The mixed liquor was wasted into these types of tanks and let settle. Then we had a hose fitted inside the tank that we could lower and then we would open a valve and slowly decant the supernatant back to the wet well. This gets the MLSS from 3000 mg/L to 10000 mg/L = 1%. This maximum can be worked out from your 3-24 hr settling test (eg. 3000 ppm X 1000ml/250 ml settling = 12000 mg/L = 1.2%). Our operators had another trick where they would add Sodium or Calcium nitrate (dissolve the powder in water in a bucket and add to the tank during decant). The addition of the nitrate to freshly decanted sludge will cause denitrification and nitrogen bubbles will float the sludge particles to the top of the tank. You then decant the supernatant from the bottom of the tank. The sludge should be better than the gravity option, if you allow some time for the solids to float and water to drain (just like the slow rake time for the float sludge in a DAF tank). I would start with a dose of say 50 mg/L = say 1 lb powder per 2500 US gal tank. You may need to decrease or increase the quantity. Monitor you supernatant for solids and if too high add more nitrate. If the solids of the float is not too good add more nitrate. I am sure you will find it a winner. However you probably also need to check that there is not too much nitrate in the supernatant - particularly if you have limited efluent discharge limits. Alternatively you would be looking at polyelectrolyte. A 2500 gal tank contains 27 kg solids at 3000 ppm MLSS. Dosing poly at around 3 to 5 kg/tonne requires 80 to 135 g of poly. Select an anionic liquid emulsion polymer (get you rep to help you) and you would use 150 to 270 ml/2500 gal. However you need a polyblend unit of appropriate size and then secondary dilution to get the polymer thin enough to dose into the pipe from the aeration tank to the sludge tank. Sounds a lot of cost and worrying about the equipment.
Regards
Grant H, Australia |
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