“non conventional anaerobic sludge digestion using a … · • mexico city water utility, wwtp...
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Fco. A. Cabeza de Vaca, J. Martínez, O. Huerta, R. Briones, y A. Noyolanoyola@pumas.iingen.unam.mx; fcabezadevacai@iingen.unam.mx
UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO
INSTITUTO DE INGENIERÍA
“NON CONVENTIONAL ANAEROBIC SLUDGE DIGESTION USING A MODIFIED UPFLOW SLUDGE
BLANKET REACTOR ”.
INTRODUCTION
• 255 m3/s Wastewater are produced in México
• 72 m3/s are treated
• 1593 municipal-WWTP´s are in operation.
• 23% are based in activated sludge process, treating up to 42% of the treated
sewage
• The activated sludge process produces waste sludge
• This residue is directly discharged back to the sewage system, disposed of on soils or placed in dumps and landfill sites
The purpose of this paper is to assess the technical suitability of a non-conventional anaerobic sludge digester that would have lower capital costs and a simpler operation.
Also, to characterize the formation of a scum layer that was constantly present during operation.
OBJECTIVES
METHODOLOGY
Schematic diagram of the pilot plant arrangement
Work Conditions
Loading rate (Bv) 2.5 kgVSS/m3d
Temp. 30°C
Flow 8 m3/d (primary and secondary sludge ratio 1:3)
HRT 2.25 d
RESULTS
0
2
4
6
8
10
12
0 100 200 300 400 500 600Time (days)
Load
ing
(kgV
SS/m
3d) .
d)..
Feeding load to the non conventional digester: average 2.41 kgVSS/m3*d
RESULTS
0.5513.6416.9430.58Sludge bed0.651.472.754.22Effluent0.663.637.1310.75Feeding
FSSVSSTSS
Volatile fraction %
Average total suspended solidsg/LSample
Solids behavior in digester
Diluted sludge
High TSS wash-out due to thick scum build-uppH 6.95; Partial to total ratio 0.53
The digester produced a thick stabilized sludge
RESULTS
995445
57 1.45 X 104
Count in sludge bed
<35Helminths eggs• viable Eggs /gTS• no viable Eggs /gTS
<300Salmonella (MPN/gTS)<2 X 106Feacal coliforms (MPN/gTS)
Pathogen requirements for all C class (NOM-004-SEMARNAT-2002)
Microorganism
Pathogens and parasites in sludge bed of non conventional digester (average of three samples)
Parasites above limits
Pathogens within limits of NOM-004-SEMARNAT-2002 for class C biosolids
RESULTS
Scum Thickness
05
1015
2025
30
460 470 480 490 500 510 520 530
Time (days )
(cm
)
Scum thickness on top of digester
Covered phase
Uncovered phase
RESULTS
a)
b)
Scum formed on the digester surface: a) uncovered stage, b) covered stage
RESULTS
0.6743.721.865.5Covered phase
0.6139.024.663.6Uncovered phase
VSSFSSTSS
Volatilefraction %
Total suspended solids g/LSample
Suspended solids behavior of the scum at the top of digester
Thick material, not yet stabilized
RESULTS
267 mg EPS / g VSS145 mg EPS / g VSSExtracellular Polymeric Substances
80 mg N / g VSS78 mg N / g VSSTotal Kjeldahl nitrogen
12 mg N / g VSS6 mg N / g VSSAmmonia nitrogen
68 mg N / g VSS72 mg N / g VSSOrganic nitrogen
48 mg FOG / g TSS71 mg FOG / g TSSFat, oil & grease (FOG)
Covered phaseUncovered phase
AverageParameter
Parameters measured in the scum
CONCLUSIONS
The UASB-type non conventional digester achieved good stabilization of the sludge bed (55% volatile fraction) and a fair thickening function, attaining a sludge bed concentration of 30g/L.
Feacal coliforms and salmonella counts were reduced to meet Class C biosolids of the Mexican Standard NOM-004-SEMARNAT-2002. However, operating conditions related with temperature and sludge retention time were not adequate for helminth eggs reduction requirements.
A major drawback of the tested non conventional design was the build up of a thick scum layer on top of the digester, with suspended solids concentration higher than 60 g/l and 60% of volatile fraction.
Fat, oil and grease, as well as proteins and exo-polimeric substances were major constituents of the scum. These compounds may be the binding element for scum stability and resistance to mechanical stress.
CONCLUSIONS
ACKNOWLEDGEMENTS
• Mexico City Water Utility, WWTP Cerro de la Estrella
• Mexican Council of Science and Technology (CONACyT)
• General Direction of Academic Affaires (DGAPA) of the NationalAutonomous University of Mexico (UNAM) grant DGAPA IN-119303
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