What Sludge Is

When we think of sludge, what automatically comes to mind is sewerage. Water carriage systems of sewerage provide a simple and economical means for removing offensive and potentially dangerous wastes from household and industry. The solution and suspension of solids in the transporting of water produces sewage. Thus, the role of solids and sludge removal at Sewage Treatment Plants is apparent. Sludge removal is complicated by the fact that some of the waste matters go into solution while others are colloidal or become finely divided in their flow through the sewage system. Ordinarily, less than half of such waste remains in suspension in a size or condition that can be separated by being strained out, skimmed off or settled out. The remainder must then be precipitated out by chemical means, filtered mechanically, or be subjected to biological treatment whereby they are either removed from the water or changed in character as to be rendered innocuous. Sewage contains mineral and organic matter in suspension (coarse and fine suspended matter), in colloidal state (very finely dispersed matter) and in solution. Living organisms, notably bacteria and protozoa, find sewage to be an abundant source of food, and their lives' activities result in the decomposition of sewage.

Sewage becomes offensive due to its own instability together with the objectionable concentration of suspended materials. In addition, the potential presence of disease producing organisms makes sewage dangerous. Removal or stabilization of sewage matters may be accomplished in treatment works by a number of different methods or by a suitable combination of these methods.

While sewage sludge is rich in nutrients and organic matter, offering the potential for applications as a biosolid (discussed below) or it has a heating value making it suitable for incineration, many industrial sludges are often unsuitable for reuse. A more common practice with industrial sludge is to try and identify a reclaim value; i.e., if the sludge can be concentrated sufficiently there may be a portion of this waste which is reclaimable or may enter into a recycling market.

Figure 1. The wire is drawn through a cleaning solution bath, which removes the copper dust from the surface of the extruded wire. The wash liquor is slightly acidic and contains a detergent so that the wire surface is clean. From this point on the wire is spooled, and then sent to another part of the operation which manufactures multi-strand telecommunications cable.

Figure 1. The wire is drawn through a cleaning solution bath, which removes the copper dust from the surface of the extruded wire. The wash liquor is slightly acidic and contains a detergent so that the wire surface is clean. From this point on the wire is spooled, and then sent to another part of the operation which manufactures multi-strand telecommunications cable.

An example is illustrated in Figures 1 through 4, which shows copper contaminated sludge from a cable manufacturing plant. In a concentrated enough form, and sufficiently dried ( to at least 60 weight % solids) this waste can be sold to a smelter as opposed to landfilling. In the latter case we face disposal costs, which include storage, transportation and tipping fees at a landfill.

Figure 2. Photograph of copper wire being drawn (cold-extruded). The wire is cold-drawn by a series of extruders until the proper gauge is achieve. During this process, fine copper particulates are abraded and attrited off of the wire surface.

If instead we can sell the waste, then we not only have eliminated these costs, but in addition have generated a positive cash flow into our operation for the sale of the waste. This is what we mean by pollution prevention practice. Of concern, however with any sludge management issue, are the costs for recovery of potentially valuable by-products as in the case of the cable plant and the costs associated with concentrating the sludge to a form that it can be handled in post processing operations.

There is an old saying among pollution control engineers that practiced pollution prevention long before it became fashionable. That saying is - 'If you see a dollar lying on the ground, it's worth your while to bend over and pick it up - but NOT if you have to break your back to do it.'

We will address some of these issues later on in this chapter. But for now, simply recognize the fact that the most basic part of waste water treatment is solids removal. Solids are removed in primary and secondary treatment tanks, but without such effective removal there is no treatment process efficiency.

Figure 2. Photograph of copper wire being drawn (cold-extruded). The wire is cold-drawn by a series of extruders until the proper gauge is achieve. During this process, fine copper particulates are abraded and attrited off of the wire surface.

Figure 3. The sludge is dried using a forced evaporation method to about 60 % solids, which makes it dry enough for bagging and transport.

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