Waste Water Treatment – Gravity Sludge De-Watering

Gravity de-watering is accomplished in a de-watering box.  This can be a roll off type box or a self-dumping hopper.   The box or hopper will have a perforated bottom plate.  Typically, a 100-mesh stainless steel wire screen is over the perforated plate.   A 20 mesh wire screen is also typical when a filter paper is to be used on top of the screen.  Other sizes of wire mesh screen are readily available.  The wire screen is typically held in a metal frame making it easy to remove for cleaning or replacement.    The wire screen may be enough by itself, if ... » Read More

Waste Water Treatment – COD vs BOD as a Measure of Waste Water Strength

BOD, Biochemical Oxygen Demand, is the standard measurement for determining the strength of waste water.    It is also a common regulatory tool used around the world to measure the strength of industrial waste water.   Most municipalities will charge a fee per 1,000 gallons of waste water when the BOD is considered to be high.    High BOD is generally anything over 1,000 mg/l where household BOD is around 350 mg/l.   Industrial waste water is often well above this threshold resulting in high fees from the sewer authority.   The sewer authority charges theses fees to cover increased cost associated with the high ... » Read More

Waste Water Treatment Coagulation and Flocculation – Importance of Mixing Energy

We were recently involved in an industrial waste water project for the removal of Molybdenum. We conducted jar testing with pH adjustment, addition of coagulant and a flocculent along with a body feed of bentonite clay. We were successful at clarifying the waste water and removing the molybdenum in the jar testing. The jar testing went so easily, that we were surprised when it failed on a pilot scale. The pilot plant was a continuous flow treatment system with three 36 gallon CSTRs (continuously stirred tank reactors) followed by simple filtration. Chemicals were added to the first tank, a rapid ... » Read More

Bentonite Clay

Waste Water Treatment Coagulation and Flocculation, Using a Bentonite Clay – Body Feed

It seems counter-intuitive, but the cleaner the waste water the harder it is to treat. When a coagulant is added to nearly clear waste water it will often form a very tiny floc known as a Pin Floc. In waste water treatment Pin Floc is to be avoided. Pin Floc tends to not settle and can cause filter problems. To produce a more robust floc that will settle and can be separated and filtered out, a Body Feed is often added. A common Body Feed for industrial waste water treatment is Bentonite Clay. Bentonite Clay is inexpensive and can be ... » Read More

Waste Water Treatment Coagulation and Flocculation – Static Mixers vs Dynamic Mixers

In waste water treatment, the term Dynamic Mixer means any mixer with a moving part, such as an impeller. This is usually a square tank with a center mounted mixer motor, shaft and impeller. Where Static Mixers are usually several sections of pipe with a convoluted insert that causes the water to change direction over and over again to achieve mixing. The mixing energy comes from the flow rate through the pipe. When adding coagulants and flocculants for waste water treatment, dynamic mixers work well for both chemicals. However static mixers work best for coagulants, but not for flocculants. Flocculants ... » Read More

Choosing the Right De-watering Box Filter Media

There are many choices for the filter media in your de-watering box. Cloth filters come on a roll that can be any width and are usually 50 yards, 100 yards or 250 yards in length. The filter cloth is typically cut by hand and laid on top of the wire screen in the bottom of the box.  The wire screen can be any mesh size but is usually in the range of is 20 to 100 mesh stainless steel. The wire screen wire can be used by itself or used to support the filter cloth. The most popular filter cloths ... » Read More

de-watering box

De-watering Box Basics

What is a de-watering box? A De-watering box will typically be a hopper with a rectangular perforated plate mounted about 8 inches off the bottom. On top of the perforated plate you can use wire mesh screen, or filter cloth. Securing the filter media is a metal frame with lifting handle. If you want to change out the filter, simply remove the four frame bolts and lift from the center mounted handle. Sample filter media When purchasing a de-watering box to separate sludge and water ask the supplier to provide some samples of filter media. Try the different filter media ... » Read More

How to Size an Oil Water Separator

If you are looking for a gravity oil water separator, here is a rule of thumb to determine the size. Multiply your estimated flow rate times 10 and that will be the minimum tank size. For example, if your flow rate is around 30 gallons per minute (gpm) you will want an oil water separator with a separation tank that is at least 300 gallons. This will give the oily water a 10 minute retention time in the separation tank. 10 minutes is usually enough time for gravity separation. If you want to be sure, collect a sample of your ... » Read More

Separating Emulsified Oils in Water

Separating Emulsified Oils in Water Oil can become emulsified in water by mechanical agitation and by chemical emulsification or both. Mechanical Emulsification of Oil in Water Centrifugal pumps often will cause mechanical emulsification. Also, any influent piping that is elevated above the surface of the oily water that is splashing into the tank will also cause emulsification and possible foaming. Oil that is mechanically emulsified will usually separate given enough time. Oil coalescing grids will also assist in separating the emulsified oils. Chemical Emulsification of Oil in Water Degreasers and soaps can chemically emulsify oil in water. Most degreasers and ... » Read More

What Do Oil Removal Claims Really Mean?

When you see a manufacturer claim that his oil water separator will remove oil and grease to below 200 mg/l or some other number, keep in mind what he really saying. They are usually basing their claim using pure water, at 65 to 70 degrees,  with some amount of clean SAE 30 weight motor oil straight from the can. Ask the vendor exactly what they are basing their oil removal claims on. Ask what percent oil was added, ask for exact SAE weight or viscosity and ask the temperature. Bench scale testing of your oily water is usually a good ... » Read More