A new method of wastewater purification bypasses primary settling to simplify treatment and reduce sludge volume.In northern regions, there are specific conditions for the treatment of sludge. These conditions invoke certain requirements for the purification of natural and wastewaters and the treatment of these sludges. The most important of these requirements are
Sewage from northern residential areas usually is diluted and has a low concentration. In many cases, biological treatment of such sewage can be performed without primary settling. This simplifies the following sludge treatment since only one type of sludge is formed (i.e., excess of activated sludge). In this case, wastewater treatment with prolonged aeration is a good technique because it reduces the increase of activated sludge. Wastewater treatment without primary settling but with prolonged aeration provides sludge volume reduction in the five to ten times range.
Based on that scheme, a new method of wastewater purification and activated sludge treatment was developed. The essence of this method is that the activated sludge from secondary settling tanks moves to the centrifuge where the water separates from the sludge. The separated "cake" is then disinfected and utilized as a fertilizer, while the fugate is used for biological treatment of wastewater (instead of circulated activated sludge or as a mixture with it). The new method's flow diagram is shown in Figure 1.
The application of this method provides dewatering of the activated sludge without the use of any flocculating agents. The whole amount of the activated sludge (or a portion of it) is moved from the secondary settling tank to the centrifuge. The amount of the activated sludge should be removed by centrifuging the yield waste activated sludge. The fugate (its content by dry matter is 70 to 90 percent of the amount moved to the centrifuge) is moved to the aeration tank. This method is especially effective for wastewater treatment plants in northern regions with capacities up to 10,000 m3/day. The application of this method in northern areas makes it possible to not have to construct covered sludge drying beds. In addition, this method does not require the use of flocculating agents; thus the cost is reduced.
This technology has been used in 20 settlements along the Baikal-Amur railway construction route in Russia. As an example, the data from a wastewater treatment plant in the town of Tynda (Russia) is shown in Table 1.
Gravitational sludge thickening in settling tanks is also widely used. However, among the drawbacks of this method is the necessity to build bulky facilities that require large tracts of land. Another problem is that the organic part of the sludge decays during long periods of thickening. This negatively affects the dewaterability process.
A new method of gravitational thickening of the mixture of activated sludge and the sludge from primary settling tanks has been developed. This method does not have the disadvantages mentioned above (Figure 2). This method also enables a considerable reduction of both capital and running costs. The essence of this method is that the quantity of activated sludge that is moved to preliminary aeration tanks exceeds its increase by a certain value that corresponds to the carryover of activated sludge from primary settling tanks. The balance of activated sludge corresponds to the sum of excessive activated sludge settled in primary settling tanks and the activated sludge that is carried over from them. The quantity of circulating activated sludge is compensated by activated sludge carried over from the primary settling tanks. The mixture's detention time in the primary settling tank is specified experimentally depending on the mixture to be settled (30 to 100 min.). The quantity of activated sludge to be fed to the preliminary aeration tank is controlled in the process of operation depending on the required quality of treatment.
This method has the following advantages:
As shown in Table 2, the new method reduces the specific resistance to filtration by four to five times.
The higher the specific resistance of the sludge, the more chemicals required for its coagulation and, therefore, the lower the productivity of dewatering equipment.
Domestic and foreign practices generally show that the preliminary thickening of sludge is a reasonable technique. The volume of sludge from low turbid water can be reduced by five to ten times, providing the optimal parameters of thickening with agitation.
Recently, mechanical dewatering on various filter presses has been widely used for hydroxide sludge of natural water. The main disadvantage of filter pressing is the necessity to add lime in the preliminary treatment of sludges with dose changes from 20 to 100 percent by CaO depending on the raw sludge quality. Lime doses during filter pressing may be reduced by adding various additives or by heating the sludge up to 70 to 80° C during the treatment.
Hydroxide sludge treatment with acids and alkalies provides raw sludge reduction as well as the utilization of some coagulant. When water purification and wastewater treatment plants are located close to one another, the use of the hydroxide sludge method may be successful.
The freezing of sludges followed by thawing alters their structure and drastically improves their dewaterability. When using this technique, the sludge should be subjected to relatively slow freezing over the whole thickness. Layer by layer freezing results in sludge lamination (i.e., when the solid phase deposits on the layer surface) and hampers its quick melting. Therefore, it is preferable to freeze sludge over the whole thickness at once. When frozen, the liquid phase of sludge is concentrated on the top layer, thus being able to be removed as ice using the chopping off technique or by decanting on the way of thawing. The height of the top layer depends on the depth of natural freezing in specific climatic conditions. It is reasonable to take in consideration the height of the top layer within 70 to 100 cm. The influence of freezing temperatures on the reduction of the sludge's specific resistance is shown in Table 3.
The dewatering of a solid phase occurs in the process of freezing due to the migration of water through the cellular walls and cells of colloids. If the freezing process goes slowly, all bound water that is capable of diffusion under current conditions has enough time to migrate into intracellular space where it becomes frozen. The pressure created during the expansion of crystallizing water promotes coagulation and aggregation of the sludge's particles in the solid phase of sludge. After melting, sludges can be dewatered in either natural or artificial conditions without the addition of chemicals.
Natural or artificial freezing of sludges may be effectively used for hard-to-dewater sludges, for example, when treating sediments of low turbid colored waters formed at factories where an aluminum hydroxide is used. The selection of a treatment technique requires conducting engineering tests that will provide maximum efficiency under specific conditions.
Further disinfection and additional reduction of water content in wastewater sludges may be reached by using biothermal processing (composting). We have developed several technologies for biothermal treatment of sludges including a device for the mixing ofsludges with filling agents, their homogenization and their saturation with air. The mixing device is 3m in diameter and 4m wide. Wooden chips, pits, straws and also some portions of mature compost are used as fillers.
Methods of biothermal sludge treatment providing compost utilization as fertilizers or bio-fuels also have been developed and used.
By using these proposed methods and comparing them with currently used techniques in northern regions, the new methods substantially reduce capital and maintenance expenses (by four to six times).
About the Author I. Turovskiy is a professor at Jacksonville University. He has a Doctorate of Science in Engineering.