Sewage Treatment

After trash and bulk contamination are removed from waste water by screening, the next step is the removal of suspended matter. This can be accomplished by several methods, the simplest of which is gravity sedimentation. Wastewater is held in a tank or vessel until heavier particles have sunk to the bottom and light materials have floated to the top. The top of the tank can be skimmed to remove the floating material and the clarified liquid can be drained off. In batch mode sedimentation, several tanks of sewage will go through the settling process before the accumulated sludge is removed from the bottom of the tank.

The settling process can be hastened by use of chemical precipitants such as aluminum sulfate. Gentle stirring with rods, another method, encourages the aggregation of a number of fine, suspended materials. As the clots of material grow larger and heavier, they sink. Suspended matter can be encouraged to float by exposing it to fine bubbles, a method known as dissolved-air floatation. The bubbles adhere to the matter and cause it to float to the surface, where it can be removed by skimming.

Another method of filtration, generally used after gravity sedimentation, is deep bed filtration. Partially processed liquid from the sedimentation tanks, called effluent, flows over a bed of graded sand and crushed coal. This material not only strains the larger particles from the effluent, but further clarifies it by removing fine particles via adhesion. The filtering material attracts these small particles of sewage by electrostatic charge, pulling them out of the main flow and resulting in significantly clearer liquid. Alternately, effluent can be filtered by a fine mesh screen or cloth, in a method known as surface filtration, or solid material can be pulled out by centrifuge.

At this point the original raw sewage has been essentially separated into two parts: sludge, or biosolids; and clarified effluent. Both parts still contain disease carrying, oxygen consuming pathogens, and need further processing. Earlier, we discussed the biological decay of raw sewage. Theoretically, both biosolids and effluent can be processed using biological treatment methods, but at this point cost considerations come into play. Biological treatment of dense sludge is time-consuming, requiring large tanks to allow complete processing, whereas that of effluent is fairly efficient. Thus, biosolids are generally processed by different methods than effluent.

After settling out, biosolids can be removed from the bottom of the sedimentation tank. These tanks may have a conical shape to allow the sludge to be removed through a valve at the tip, or they may be flat bottomed. The sludge can be dried and incinerated at temperatures between 1,500–3,000°F (816–1,649°C), and the resulting ashes, if non-toxic, can be buried in a landfill. Composting is another method of sludge disposal. The biosolids can be mixed with wood chips to provide roughage and aeration during the decay process. The resulting material can be used as fertilizer in agriculture. Properly diluted, sludge can also be disposed of through land application. Purely municipal sludge, without chemicals or heavy metals, makes a great spray-on fertilizer for non-food plants. It is used in forestry, and on such commercial crops as cotton and tobacco. It must be monitored carefully, though, so that it does not contaminate ground water.