ANAEROBIC DIGESTION OF SEWAGE SLUDGE USING SEQUENTIAL REACTORS

The objective of this work was the implantation and evaluation of three sludge anaerobic digestion systems. The system A configuration was defined by five continuous flow sequential anaerobic reactors (SAR) with effluent recycling. System B had the same layout, but recirculation was substituted by mechanical mixers installed in each reactor. System C was composed by only one continuous flow stirred tank reactor. The systems were evaluated by weekly monitoring of physical-chemical parameters. Samples were taken from influent, effluent and intermediary ports during an operation period of 115 days. During the most stable operational period (PHASE II), the results indicated total solids removal rate of 95, 40 and 29%, and volatile solids removal rate of 95, 34 and 22%, for System A, B and C, respectively. The sequential anaerobic reactors (System A and B) presented better performance than conventional digestor (System C)


INTRODUCTION
Conventional wastewater treatment plants always generates as by-product the sludge formed in the primary and secondary settling tanks, which needs additional treatment for complete stabilization.
Although there is a trend to apply "lesser sludge production " technologies, it is still very frequent the application of conventional systems generating huge volume of sludge.
The final adequate disposal of sludge is a problematic stage in the operational process of a sewage treatment plant and that, normally, has been neglected.The total costs of the sludge treatment units may represent one third of the total investment (VESILIND, 1974) or 50% of the operational budget (BETTIOL & CAMARGO, 2000).
Research in the direction to optimize anaerobic digestion through technological development of "new conception systems" has not been explored, stimulating the proposal of this work consisting of installation and evaluation of a sequential anaerobic reactor system (SAR) treating sludge from primary and secondary sedimentation tanks of a conventional sewage treatment plant.This proposal is based upon the possibility of improvement of system hydrodynamic conditions, leading to an optimization of mixture condition and better contact substrate-biomass.Also, biochemical reactions involved in the anaerobic process may be favored by the creation of distinct environment conditions in the diverse reactors, providing better process stability and higher system performance.The main objective of this work was to develop comparative studies of the sludge stabilization process using continuous flow sequential anaerobic reactors and conventional continuous flow stirred tank reactor determining design parameters and optimizing operational procedures.

MATERIALS AND METHODS
This research was developed at the sewage treatment plant Carioba, located in the city of Americana -SP -Brazil.The pilot plant, illustrated in Figures 1 and 2, was composed by an equalization tank of 2,500L volume, which fed 3 anaerobic reactors systems.The first system (A), was composed by five continuous flow sequential anaerobic reactors (SAR), each one with a 1,000L capacity with a recycling line from the fifth to the first reactor.The second system, also a "SAR" system, was similar to the first one.The difference of the previous one was the use of slow mixers installed in each reactor.Finally, the third system (C), was composed by single continuous flow stirred tank reactor with 5,000L capacity.The influent in each reactor was introduced near the bottom in order to achieve the best possible even distribution in the tanks.
The pilot plant (equalization tank and reactors) was assembled using water storage tanks made out of fiberglass, promptly available in hardware stores.
The pipeline, valves, and sampling ports were manufactured in PVC, also available in hardware stores.A submerged pump (KSB-KRT Drainer 1500) was utilized to pump sludge to equalization tank.From there, positive shift pumps (Netzsch-3.NU.06), proper for high solids concentration liquids, fed the 3 systems and used to recycle the effluent in System A.
The evaluations of the 3 systems were carried out analyzing the following parameters: Total Solids (TS), Volatile Solids (VS), Fixed Solids (FS), pH, Volatile Fatty Acids (VFA), Partial Alkalinity (PA) and Total Alkalinity (TA).TS, VS, FS and pH determination were carried out according to the Standard Methods for Examination of Water and Wastewater (APHA, 1990).VFA determinations were carried out according methodology proposed by DILALO & ALBERTSON (1961).TA and PA determination were carried out according methodology proposed by RIPLEY et al (1986).The sampling ports are indicated at Figure 1.
The systems were operated during115 days under hydraulic retention time (HRT) of 30 days, including PHASE I (Start-up -55 days) and PHASE II (60 days).In future studies will be also evaluated lower hydraulic retention times (20 and 10 days).

RESULTS AND DISCUSSION
The average values for the analyzed parameters are shown in  From 55 th day of operation, the pH values have indicated a clear trend to stabilization in values next to 7.0 for all systems.This tendency of pH increase was followed by the increase of alkalinity (total and partial) and by the reduction of volatile fatty acids.This behavior allowed identifying the change of the Start-up period (PHASE I) to the operation period (PHASE II).The hydraulic retention time for this operation period was 30 days.Up to the 55 th day operation, as observed in Table 1, the parameters had shown low variability in samples taken from influent and effluent.
Only system A, which has no mixers presented considerable average removal for TS (74%) and VS (79%).The solids sedimentation in the reactors (with no mixers) may be the reason for that.
The results of PHASE II, shown in Table 2, indicated an improvement in the performance of the systems A, B and C, compared to PHASE I.All systems presented TS, VS, and VFA reductions and increase of pH, TA, and PA.
During PHASE II, system A presented average removal of TS and VS up to 95%.The better performance of system A may be observed by the evolution of TS and VS concentration during system A operation.Figure 3 illustrates it.However, the sludge accumulation at the bottom of the reactors must be considered.This fact does not allow a conclusive evaluation for system A performance for a longer period.
System B had better performance than system C removing TS and VS.This is showed in Figures 4 and 5. Table 2 indicates the system B had TS and VS removal efficiency about 10% higher than system C.
From Figure 6 it may be observed the high data spreading for TS and VS (influent and effluent) for systems A, B, and C, indicating a non-stable operation period (Start-up).However, Figure 7 shows low data spreading for TS and VS (effluent only), indicating a trend for stabilization in systems effluents.
From Figure 7 may be observed better performance for sequential anaerobic reactors (Systems A and B) compared to conventional digester (System C) during the most stable operation period (PHASE II).

CONCLUSIONS
The evaluation of system A, B e C, operated approximately 115 days (including 55 days of Startup period) allow to conclude that sludge digestion using sequential anaerobic reactors (SAR) presented higher performance than the conventional anaerobic digester for 30 days hydraulic retention time.
The development of simplified technology utilizing the SAR system seems to be viable.Nevertheless, more studies testing other hydraulic retention time are necessary in order to determine design parameters and to optimize operational procedures.

FIGURE 6 .FIGURE 7 .
FIGURE 5. Total Solids and Volatile Solids concentration during operational period in System C.