Q. We have a sand filter that is used to treat our plant wastewater before it’s discharged to the sanitary sewer. We are not permitted to discharge the backwash water from the sand filter, and we have been paying to have it hauled off. We would like to eliminate the expense of having the backwash water taken off site. Are there any options for us to consider?
A. Yes, there are several options you can consider. However, since you want to reduce by as much as possible your costs of handling the backwash water, we’ll suggest the simplest and least expensive approach.
Direct the backwash water to a holding tank. Attach a small pump to the tank. Transfer water in batches from the tank and filter the water using a multi-layer bag or cartridge filter. Effluent from the filter should be clean enough to put into the sanitary sewer. However, before putting the filtered water into the sanitary sewer, consult your local regulatory authorities to make sure they will allow the discharge.
Dirty bags or cartridges can be disposed of at lower cost than the backwash water because the weight and volume are less. Consult your licensed disposal company to determine whether the suspended solids in the bags or cartridges can be handled as non-hazardous material.
Some of the suspended solids in the holding tank, may be heavy enough to settle to the bottom. Periodically, you’ll need to drain off the sludge that has accumulated, so you’ll want to equip the tank with a drain nozzle and valve. You’ll handle disposal of the sludge in the same way you handle disposal of the bags or cartridges.
Q. We have just put a new water well into service. Water coming from the well is discolored. How can we correct the problem?
A. Water can extract color from underground vegetation or other organic material with which it comes in contact. Color may also indicate the presence of iron bacteria. Have a sample of water tested for the presence of iron. If the well water contains iron, all or much of the problem can be corrected by installing an iron removal filter.
If the water does not contain iron, have a water laboratory measure the amount of color in a sample of the well water both before and after they have poured the water through filter paper.
If the color has been eliminated from the filtered water or if it has been reduced to an acceptable level, filtration alone will correct the problem. If further color reduction is needed, adding a coagulant to the water (usually aluminum sulfate) followed by filtration may achieve the desired result. If color is still not reduced to an acceptable level by filtration, a granular activated carbon filter will likely be required.
Depending on the origin and size of the color particles, color removal may require one or more of the following treatment methods:
- Iron removal filter
- Slow rate media filter (with or without coagulation)
- Granular activated carbon filter
Q. We installed a sand filter at our plant about six months ago and recently we’ve started seeing greatly reduced flow out of the filters. What’s causing this?
A. The most frequent cause of reduced flow from sand filters is hardening of the filter media. When this happens, the sand acts like a barrier and blocks the normal flow of water. It’s likely that this condition has resulted from inadequate backwashing of the filters. Backwashing may be inadequate because it’s
- not being performed as frequently as it should be
- the amount of water being used for backwashing is inadequate
- the duration of the backwash is too short, or
- there is insufficient backpressure on the discharge manifold to achieve needed backwash water velocity.
If coagulants or flocculants are being added to the water before the filters, the floccs may be sticking to the sand, making it difficult to flush them out of the filter during backwash. As a result, the filter media is not being cleaned thoroughly. Over time, this too can cause hardening of the filter media and resulting reduced flow.
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