anaerobic processes in activated sludge
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~ PergamonPII: S0273-1223(98)OOI~8
Wat. Sci. Tech. Vol. 37. No. 4-S. pp. 6OS-608. 1998.1998IAWQ Published by Elsevier ScIence LId
Printed m Great Britam.0213-1223198 $19'00 + 0'00
ANAEROBIC PROCESSES IN ACTIVATEDSLUDGE
Dirk de Beer*, Andreas Schramm*,Cecilia M. Santegoeds* and Helle K. Nielsen**
Max Planck Institute for Marine Microbiology, Microsensor group. Celsiusstrasse1,28359 Bremen. Germany University ofAarhus. Department ofMicrobial Ecology, Ny Munkegade 540.DK-8000 Aarhus, Denmark
We found anoxic zones in aerated activated sludge flocs. and demonstrated denitrification under normaloperating conditions. Sulfate reduction was not found. Micro-environments and microbial conversions inflocs from bulking and non-bulking activated sludge were determined with microsensors for H:zS. 02> NOrand NOr, Denitnfication and sulfate reduction rates were measured with ISN_ and 3SS-lracer techniques.We showed thai under normal reactor conditions (ca. 20% air saturation) anoxic zones develop within flocsallowing denitrification. The demtnfication rates amounted to 40% of the rates under anoxic conditions. At100% atr saturation no anoxic zones were found and no denitrification occurred. However. in flocs frombulking sludge (at 20% air saturation) anoxic zones were absent and denitrification did not occur. In bulkingsludge only at total anoxia was denitrification found. Confocal microscopy showed that flocs from bulkingsludge were much looser than those from non-bulking sludge. The absence of anoxic zones and ofdenitrification was allributed to the open floc structure. allowing advecllve oxygen transpon.
Sulfate reduction was not detected in any of the sludges tested by microsensors or by tracer techniques evenunder anoxic conditions. This indicates that the sulfur cycle (sulfate reduction and sulfide oxidation) does notplaya role in mineralization processes and bulking in activated sludge. Preliminary molecular work (in situhybridizatIOn with the 16S-rRNA probe SRB38S) indicated the presence of small amounts of sulfateredUCing bacteria in all sludges. Either the probe is not specific or lhe sulfate reducers present are not activeunder reactor condilions. 1998 IAWQ. Published by Elsevier Science Ltd
Activated sludge; micro-environment; oxic; anoxic; microsensor; sulfate reduction; denitrification; confocalmicroscopy.
In wastewater treatment plants microbial communities are retained in the reactor by accumulation in abiofilm or by gravity settling in flocs or aggregates. In these structures micro-environments can develop dueto mass transfer resistance within the biomass layers. Microsensors have been successfully applied tocharacterize the micro-environments in biofilms. aggregates and sediments. Due to mass transfer limitationsanaerobic zones have been found in well aerated biofilm systems. Thick biofilms and sediments were foundto be stratified in zones where different electron acceptors (02, NOf. S04-2) are reduced (Kuhl andJorgensen, 1992; Revsbech et al., 1988). With microsensors, denitrification and sulfate reduction rates couldbe determined.
606 D. DE. BEER et al.
The presence of anaerobic proeesses in activated sludge operated under aerobic conditions may haveimportant treatment implications. Coupled nitrification and denitrification within one floe is possible.Removal of xenobiotics can require both aerobic and anaerobic proeesses (Gerritse and Gottschal, 1992).Furthermore, anaerobic mineralization oecurs with lower biomass yield than aerobic proeesses, with afavorable effect on the sludge production. A previous study on activated sludge reported the presence ofsulfate reducers and methanogens (Lens et ai, 1995), which was considered indicative of anoxic zones.However, direct proof for the existence of anoxic zones and anaerobic conversions was not reported. Wecombined microsensor techniques, physiological tests and molecular techniques to investigate the microenvironment inside floes, to determine the presence of anaerobic proeesses under aerobic conditions and toassess whether anaerobic microorganisms are present. The use of microsensors in activated sludge floes hasbeen hampered by the difficulty of immobilizing the floes. We used a newly developed flow-cell, whichallows immobilization of fragile floes in a laminar water jet.
Activated sludge was obtained from municipal wastewater treatment plants in Prague, Czech Republic(WWTP) and in Bremen, Germany (WWTB), and from two lab-scale sequencing batch reactors. The 5 ISBRs contained 2-3 g dry weightll sludge, originating from the Prague municipal wastewater plant. Onereactor (RI) was operated at 2 cycles per day (4 h anoxic, 8 h oxic), the other (R2) was operated at I cycleper day (23 h oxic, I h anoxic). Both were fed with artificial wastewater containing 1000 mg coonpeptone. 500 mg coon ethanol, 500 mg coon acetate, 20 mgn P-phosphate and 4 mM NaHC03 in tapwater containing 0.8 mM sulfate. Additionally, 400 mg nitrate-N was added to Rt.
Microprofiles were recorded in a 5 cm diameter Plexiglas cylinder. A stoeking was glued 8 cm from theinlet. With a large pipette floes were placed in this flow cell. Medium was pumped upwards with such aveloeity that floes were suspended about I mm above the netting (Plough and Jorgensen, 1997).Microsensors for oxygen (Revsbech et aJ, 1988), H2S (Jeroschewski et aJ, 1996) nitrate and nitrite (de Beeret ai, 1997) were used for profiling.
Denitrification and sulfate reduction rates were measured in a I I batch reactor. The bulk liquidconcentration was aerated (by recycling the head space gas) to 0%, 20% and 100% air-saturation. Theoxygen concentration in the bulk was monitored continuously. For measurements of the denitrification rate15N-nitrate was added (0.1 mM). Regularly gas was sampled from the head space for 15N2 analysis (Nielsen,1992). Sulfate reduction rates were determined using 35S-sulfate. Samples were taken over time and fixed in20% Zn-acetate for later analyses of reduced sulfur compounds (Jorgensen, 1994). The aggregate structurewas studied with confoeal scanning laser microscopy after staining with FITC.
RESULTS AND DISCUSSION
The floe size was strongly influenced by hydrodynamics, and thus very sensitive to the intensity of aeration.When an activated sludge sample was aerated sufficiently vigorously that air saturation was approached, allfloes were smaller than 0.5 mm. However, when floes were aerated to 20% air saturation the hydrodynamicregime was much milder and larger floes of 0.5 to 1.5 mm were formed. We have observed such large floesin the aeration basins of municipal plants. We have not quantified the floe size distribution under differenthydraulic regimes, but a floe size of 0.5-1.5 mm seems to be realistic.
Microsensor measurements. The oxygen microprofiles showed that the oxygen concentration inside the floesof a non-bulking sludge was lower than in the bulk (Fig. IA). If the bulk liquid is air saturated floes are fullypenetrated by 02' Then the nitrate profile indicates no consumption. However, at 20% of air saturation(common in activated sludge plants) anoxic zones develop inside floes. Under these conditions the nitrateconcentration in the floes is lower than in the bulk, indicating denitrification.
In all floes studied nitrite was present but at insignificant concentrations 2~). A clear difference wasobserved between floes from bulking- and non-bulking sludge. In floes from bulking sludge the oxygen
Anaerobic processes in activated sludge 607
profiles were not very steep (Fig. tB). At 20% air-saturation no anoxic zones developed in the floes. Onlyunder total anoxic conditions did nitrate consumption in the floes result in a steep nitrate profile. The nitrateconcentration in the floes was close to the bulk level. In none of the sludges was sulfide production detected(data not shown).
o (mM) o (mM)11
0.1 0.2 0.30.2 0.3 0.0
1 : 9 """(II ~~.j
0.06 0.08 0.06 0.08 0.10NO -(mM) NO'(mM)
Figure I. 02 (-0- and -.-) and N03- (-0- , -[ ,11- and .[Xl) micro profiles in non-bulking and bulking sludgenoes. Measurements were perfonned at I()()% air saturation in the bulk liquid (open symbols), 20% of air
saturation (closed symbols) and under anoxic conditions (-[Xl-).
Tracer experiments. The microsensor measurements were accompanied by physiological tests for sulfatereduction and denitrification (Table I). All tracer e~periments were in good agreement with the microsensormeasurements.
Table I. Denitrification rates* in activated sludge. measured with the ISN stable isotope technique underdifferent aeration conditions (% air saturation)_ - ___ _._ _ ___._--_.-
....~!.!-!.~.g!?J.Y.P.!?..\.!:.QD.~.!!I.Q!! J.QQr.~ t.Q.~_...Jl..r.!_RI (non-bulking) 0.09 0.97 2.69R2 (non-bulking) 0.11 0.5 I 1.67WWTB (bulking) 0.00 0.01 1.43
.........w~.P' ..(~y'l.~!!:1gt Q:9.1 9.:.Q.~ R:.~.~ .*numbers are in Ilmol Ni(g biomass x minute)
For all sludges the ISN-isotope tests showed very low denitrification rates at air..saturation (100%), anddenitrification under anoxia (0%). At moderate aeration (20%) significant differences were observedbetween bulking and non-bulking sludge floes: in normal floes denitrification was high, in bulking sludgedenitrification was absent.
In none of the sludges could sulfate reduction be demonstrated even under total anoxia and in the absence ofnitrate. Thus in the investigated sludges sulfate reduction is insignificant, either because sulfate reducingbacteria are absent, the conditions are unsuitable or the suitable conditions must be imposed for a longerperiod to allow the sulfate reducers to build up.
Molecular analysis. The DGGE patte