“non conventional anaerobic sludge digestion using a … · • mexico city water utility, wwtp...

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