Combined Anaerobic/Aerobic Digestion: Effect of Aerobic Retention Time on Nitrogen and Soiids Removal Jongmin Kim^*, John T. Novak^
ABSTRACT: A combined anaerobic/aerobic sludge digestion system was studied to determine the effect of aerobic solids retention time (SRT) on its solids and nitrogen removal efficiencies. After the anaerobic digester reached steady state, effluent from the anaerobic digester was fed to aerobic digesters that were operated at 2- to 5-day SRTs. The anaerobic system was fed with a mixture of primary and secondary sludge from a local municipal wastewater treatment plant. Both systems were fed once per a day. The aerobic reactor was continuously aerated with ambient air, maintaining dissolved oxygen level at 1.1 ± 0.3 mg/L. At a 4-day or longer SRT, more than 11% additional volatile solids and 90% or greater ammonia were removed in the aerobic digester, while 32.8 mg-N/L or more nitrite/nitrate also was measured. Most total Kjeldahl nitrogen removal was via ammonia removal, while little organic nitrogen was removed in the aerobic digester. Water Environ. Res., 83, 802 (2011). KEYWORDS: combined anaerobic and aeration digestion, continuous aeration, solids reduction, nitrogen removal. doi:10.2175/106143011X12928814444970
Introduction Excess nitrogen in watersheds is one of the major causes of eutrophication and water quality deterioration. In this regard, the U S Environmental Protection Agency (Washington, D.C.) (U.S. EPA) has regulated the effluent nitrogen level in treated wastewater, and this has been a driving force for research to biologically remove nitrogen during wastewater treatment processes. Nitrogen in wastevrater primarily exists as organic and ammonia-nitrogen, and organic nitrogen is converted to ammonia-nitrogen during treatment. Once organic nitrogen is converted to ammonia, it can be removed by combined nitrification/denitrification processes. Simultaneous nitrification/denitrification systems have drawn interest (Grady et al., 1999; Van Niel et al., 1992; Wang et al., 2007) because these systems do not require an additional anoxic zone for denitriñcation, and organic matter in wastewater can be used as an organic source for denitrifiers. Grady et al. (1999), using computer simulations, showed that the performance of sequencing batch reactors (SBRs) could be optimized for greater ammonia
' Infilco Degremont Inc., Richmond, Virginia. ^ Department of Civil & Environmental Engineering, Virginia Tech, Blacksburg, Virginia. * Infilco Degremont Inc., 8007 Discovery Dr, Richmond, VA 23059; email:
[email protected].
802
removal and low nitrate in the efiluent by manipulating the aeration cycle. They concluded that the optimum aeration fraction was 60 to 70% of the total cycle time of the SBR. Zeng et al. (2004) also showed that simultaneous nitrification/denitrification was possible in a laboratory-scale SBR operated in an alternating anoxic-aerobic mode v^fith a low dissolved oxygen concentration. In their study, only 2% of oxidized ammonia was found as nitrite or nitrate, while 98% was emitted from the system as gas. These authors also observed that most nitrogen removal occurred via nitrite, not nitrate, and the end product was mostly nitrogen oxide rather than nitrogen gas. Some research has shown that microbes, such as Alcaligenes faecalis, T. pantotropha, and Pseudomonas sp. are capable of heterotrophic nitrification and denitrification under aerobic conditions (Van Niel et al., 1992). A. faecalis is a nitrifier commonly found in soil and activated sludge. The deference between the amount of oxidized ammonia and the amount of oxidation products in simultaneous nitriñcation/denitrification systems may be explained by the abundance of heterotrophic nitrifiers that are also capable of aerobic denitrification. Experimental results showed that approximately 6 to 12% of nitrite in the aerobic zone could be removed by denitrification in the same aerobic system operated at a dissolved oxygen concentration less than 1 mg/L and a low ratio of chemical oxygen demand to total Kjeldahl nitrogen (COD/TKN = approximately 2.9) (Wang et al., 2007). A study performed by Kumar (2006) showed that a sequential anaerobic/aerobic (ANA/AER) sludge digestion system resulted in more than 10% additional volatile solids reduction compared with the volatile solids reduction by a single-stage anaerobic digestion system. The aerobic digester accounted for 10 to 15% of the total volatile solids reduction from the combined system when the aerobic digester operated at a dissolved oxygen concentration of 3 mg/L for 3-, 6-, and 9-days SRTs. With additional volatile solids reduction, the aerobic digester removed approximately 50% total nitrogen (TKN + oxidized nitrogen) from the anaerobic digester effiuent. Kumar (2006) speculated that the most nitrogen removal in the aerobic digestion system was the result of simultaneous nitrification/denitrification, because little oxidized nitrogen was measured from the aerobic digestion effluent. A similar result also was reported by Novak et al. (2009), who observed 62% overall volatile solids reduction for a combined ANA/AER system operated at a dissolved oxygen concentration of 2.5 to 3.0 mg/L for a 15-day anaerobic and 5day aerobic SRT. The same combined digestion system also removed 64.5% TKN. Water Environment Research, Volume 83, Number 9
Kim and Novai< Wastage (0,5L/day) •^ — —* 1
Feeding (1 L/day)
Wastage
(UJday)
Table 1—Characteristics anaerobic system.
ot t h e e f f i u e n t
from
the
Value
-AER - Various SRT
t [
t
t
Aeration
t !
Figure 1—Overall digester setup. Study Baci(ground A wastewater treatment plant (WWTP) in Philadelphia, Pennsylvania, was considering renovation of their biosolids storage facilities so they could be converted to aerobic digesters. These facilities had a capacity to provide a retention time of approximately 2 days, which was smaller than the digester configuration of previous studies by Kumar (2006) and Novak et al, (2009), Therefore, the l^cility wanted to determine if and how much additional tankage would be needed to provide postanaerobic aerobic digestion to achieve additional solids destruction and ammonia removal up to 90%, In this regard, the effiuent from an anaerobic digester was aerobically digested over a range of SRTs from 2 to 5 days to compare the following: • Additional solids removal efficiencies in the aerobic digestion systems operated under continuous aeration, and • Nitrogen removal efficiencies in the aerobic digestion systems operated under continuous aeration.
Methodology Digester Setup. A process flow diagram for the laboratory digestion system is shown in Figure 1, The anaerobic digester was prepared using a high-density polyethylene brewer tank (Model No, fl5b. The Hobby Beverage Equipment Company, Los Angeles, California), A rubber gasket and silicone grease were used to ensure gas-tight sealing. Continuous gas mixing was applied by circulating headspace gas at 1200 mL/min to the bottom of the reactor using a peristaltic pump (Model No, 755370, Cole Parmer, Vernon Hills, Illinois), A gas bag also was installed on the top of the reactor to alleviate excess gas pressure and measure gas production. The total sludge volume was 30 L, and the SRT was 20 days. The SRT was the same as the hydraulic retention time, as the anaerobic reactor was completely mixed. Inoculation was carried out by adding 6 L of anaerobically digested biosolids from a WWTP in Philadelphia, Pennsylvania, The rest of the sludge volume (24 L) was filled by adding 1,5 L of feed sludge per day. Once the sludge volume reached 30 L, wasting started. The reactor was kept in a constant-temperature room at 35 C throughout the study. The feed sludge was a mixture of primary and secondary sludges from the municipal WWTP in Christiansburg, Virginia, The target total solids concentration of the feed sludge was 2%, Once steady state (standard deviation of volatile solids reduction
