DPMC LAGOONS

   Dual-power, multicellular (DPMC) aerated lagoons can be classified as high-performance systems. The evolution and design of DPMC lagoons are discussed elsewhere (Rich 1982a, 1982b, 1985, 1996, 1999). Basically, the lagoons consist of a reactor cell with a retention time of 1.5 to 2.5 d which is aerated at a level that will maintain most of the solids in suspension, followed by three settling cells in series, each with a retention time of 1 d and aerated at a level that will permit the settleable solids to settle yet maintain dissolved oxygen in the water column. By aerating the first cell at a level that will keep most solids in suspension, sufficient turbidity is maintained in the cell to minimize the growth of algae. Consequently, growth potential for such organisms exists primarily in the settling cells. Many DPMC systems are presently in operation in South Carolina and other regions with similar climates. Although nitrification will occur in most DPMC systems, especially in the warmer months, such nitrification is erratic and should not be depended upon to meet an effluent limit. The lagoons are strictly for carbon removal in temperate climates.
 

    Early in the acceptance of DPMC lagoons as secondary treatment systems, concern was expressed as to the frequency at which sludge would have to be removed from the systems and how would the accumulating sludge with the resulting reduction in retention time impact performance. In absence of long-term operating performance, reliance had to be placed both on an assumed fraction of nonbiodegradable solids in the influent wastewater and an estimate of the sludge solids percent of the accumulated sludge in order to predict accumulation rates (Rich 1985). No attempt was made to predict the effect that reduced retention time would have on performance.

SLUDGE ACCUMULATION

   One of the earliest DPMC lagoons to be constructed is located in Berkeley County, South Carolina. As is typical for these lagoons, the lagoon basin is divided into cells by floating curtain walls, and the water depth in the lagoon is 3 m (10 ft). Placed in operation in 1986, the flow into the lagoon has remained at about 40 percent of the design flow of 17.52 L/s (0.400 MGD). Sludge has never been removed from the system. Figure 1 illustrates the sludge accumulation depths in the four cells as of June 2002. Each depth is the average of five in situ measurements at different locations within the cell. The accumulation of sludge found in the first cell can probably be attributed to a combination of factors: regions of lower turbulence levels between the surface aerators, influent sand, and leakage from the second cell under or around the floating curtain wall separating the first and second cells. Distribution in the settling cells is what one would expect, accumulation varying with distance from the first cell. Based on cell size, estimates indicate that sludge now occupies approximately 58 percent of the lagoon volume. The long-term effluent record for the lagoon is shown in Table 1. The effluent statistics are based on a log-normal distribution of data obtained from monthly discharge monitoring reports submitted to the state regulatory agency. The statistics at the top of Table 1 extend from January 1990 to June 2002, a period of 12.5 years. For comparison, averages of the effluent TSS and BOD5 are given below for the recent 2.5 year period extending from January 1999 to June 2002. It appears that sludge accumulation has had little or no effect on performance.

 Long-term sludge accumulation rates for two DPMC systems are presented in Table 2. For the Berkeley County lagoon, the rate measured for the 16 year period is less than that for the 7 year period. Such reduction can be explained by the further consolidation of the sludge resulting from additional stabilization and increasing weight of sludge on the bottom solids. For domestic wastewaters, these accumulation rates provide estimates with which the sludge storage requirements can be determined in the design of DPMC lagoons. Alternatively, storage requirements can be determined from estimates of typical sludge solids percentages and estimates of the nonbiodegradable solids in the settling sludge.

SLUDGE SOLIDS PERCENT

   Properties of sludges measured at the bottom of four South Carolina DPMC lagoons are presented in Table 3. Values in the table were obtained from samples taken from the bottom of the lagoons with sludge samplers. All four lagoons treat domestic wastewater. The variation observed in sludge solids percentages probably is due, for the most part, to sand that is carried into the lagoons by the sewage. This is reflected in the variation observed in the volatile solids percentages.
Sludge properties will vary from top to bottom of the sludge layer. Bryant (1983) found that the percent solids of a deposit of activated sludge which was added to on a weekly basis increased from 1.2 to 2.4 percent in 15 weeks. Rich and Conner (1982) in another study involving activated sludge solids added intermittently to a deposit found that the percent solids at the bottom of the deposit was 3.25 percent at the end of 42 weeks. Evidently, for such deposits the percent solids varies in a parabolic relationship with time.

DISCUSSION

   At the present, insufficient information is available to estimate sludge accumulation rates based on percent sludge solids and percent volatile solids. Needed are these properties at the top of the sludge and at least one intermediate depth. However, sludge accumulation rates based on volume can be estimated using the information given in Table 2. Furthermore, in spite of the relatively short hydraulic retention time provided for in DPMC lagoons, it appears that the performance is unaffected by significant sludge accumulation.