Domestic Wastewater Treatment

In Sub-optimal Temperature (13 + 1.8 ° C) in a Piston Flow Anaerobic Bioreactor at Laboratory Scale

By: Ivan R. H. Medina / Walter Q. Mamani

Spanish version: Tratamiento de aguas residuales domésticas a temperaturas sub-optimas (13+1.8°C) en un bioreactor anaerobio de flujo pistón a escala laboratorio

Report submitted for publication in the magazine “Technology for an Integrated Latin America” Submitted for presentation at the Seventh National Congress of Sanitary and Environmental Engineering, Santa Cruz, Dic. 1995.

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Summary

The UASB concept, sludge blanket reactor and upflow is one of the most studied systems in Latin America and the Caribbean within the unconventional anaerobic processes applied to the treatment of domestic sewage. The experiences at laboratory and real-scale show that has a quite satisfactory behavior for tropical and sub-tropical climates with temperatures above 20 ° C. This situation makes several Latin American experts consider the need to devise, develop and adapt new concepts within of non-conventional anaerobiosis as viable answers to the problems of pollution from domestic sewage in the Andean countries with temperatures well below 20 ° C.

 
 

One of the most viable alternatives for climates between 10 and 20 ° C, is the Anaerobic Reactor with upflow piston ARP, developed at the University of the Andes by Alvaro Orozco. The ARP concept is extremely important, since it is an effective, safe, compact and inexpensive (competitive and complementary to impoundment) Latin Amrican technological response for the deteriorating quality of most of the water courses in the Andean zone.

This exposes the Domestic waste water treatability study results at a sub-optimal temperatures in an anaerobic piston upward flow bioreactor, at laboratory, called RAP-100 scale.

Control parameters and operational characteristics of the commissioning process in which granulation is achieved (immobilizing bacterial through flocs) using septic tank sludge as seed are summarized.

Results of 51 days of operation are presented, under an actual daily flow profile, simulated at laboratory level, achieving the RAP-100, to operate with maximum and minimum peak hours of wastewater flow, in the same proportion and frequency as a treatment plant (without a regulation tank) under real conditions. During this period it has worked with an average hydraulic retention time of 8 hr. ascensional speed 0.313 + 0.115 m / h, temperature 13 + 1.8 ° C, obtaining removal efficiencies of chemical oxygen demand COD, biochemical oxygen demand BOD BOD 5 and most probable number of fecal coliform NMPCF: 77.5 + 5.5%, 84.5 + 1.5% and 67 + 19%, respectively. The kinetic constants K and K2 of COD removal, evaluated for the Orozco’s kinetic model 1989, have values 0.575 [mg. COD / mg. SSV. Dial and 0.094 [mg.DQO / mg. SSV] 14.7 ° C, kinetic constant Ka of removal NMPCF, evaluated for the Polpraset and Hogan kinetic model is 5.2 + 2.8 [1 / day] at 14.7 ° C.

Profile development within the RAP mud is also shown, being estimated the rate production  of mud by organic matter removed. Finally is shown that the chambers 1 and 2 of RAP-100, have relatively critical control parameters and therefore can be used as “control pilot chambers”.

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Keywords

Anaerobic Reactor with upflow piston, bacterial immobilization, control and operation parameters, real daily flow simulated profile, hydraulic retention time, ascensional speed, removal efficiency, kinetic constants, kinetic model, control pilot chambers of the ARP 100.

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Objectives

The objective of this work is to obtain information on the treatability of domestic wastewater in an anaerobic reactor with upflow piston at laboratory level, at suboptimal temperatures (13 ± 1.8 ° C): load profile, control and granulation phenomenon during commissioning, optimal hydraulic retention time, efficiencies and kinetic removal constants COD, BOD 5 and NMPCF, etc.

 
 

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Introduction

The Anaerobic Reactor with upflow piston “ARP” is an unconventional anaerobic bioreactor developed at the University of the Andes, this technology has been highly satisfactory operating at temperatures below 20 ° C, such as: low hydraulic retention time, high removal deficiencies of biochemical oxygen demand, etc., so it can be expected that the validation of its technology, raise alternatives for low-cost solution for biological treatment of domestic wastewater at sub- optimal temperatures.

The ARP, developed at the University of the Andes, has 55% of its volume occupied by upflow. From bibliographic analysis and considering that the upward flow is one of the main factors for the formation of granular sludge Lettinga- 1985 (mud responsible for the higher clearance rates presented in these systems), we propose to introduce a modification that achieves the entire volume of the RAP, is occupied by upstream, thereby creating more “active zones” to promote the development of granular sludge, this reactor will be denominated for comparative purposes RAP-100.

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Materials and Equipment

Anaerobic Reactor with upflow Piston “APR-100”:

The reactor was constructed of glass with eleven chambers and a usable volume of 25 lt, as a means of high porosity to promote the separation of sludge – gas – liquid was used No. 25 curlers, placed on top of each chamber. The upward flow throughout the reactor is accomplished by a PVC sewer system as the only connection between chamber and chamber. (Fig. 1).

Dosing equipment for conservation and continuous feed of wastewater:

The Dosing equipment and conservation of wastewater to the reactor consists of a hermetic glass tank of 26.7 l capacity, with frosted glass cover where an air inlet control device is connected via capillary tube, the wastewater is loaded to the tank every 48, 24, 12 and 6 hours, according to the RAP-100’s hydraulic retention time.

This residual water tank is located inside a refrigerator and maintained at 4 ° C and has a magnetic stirring system to prevent settling of suspended solids. The equipment is fully calibrated for different capillary tubes (with similar diameters) and under working conditions at laboratory level (fig. 2).

Tempering device of wastewater fed to the ARP:

For tempering residual water that comes out at 4 ° C of refrigerator before its entry into the ARP, a heat exchanger was built with glass and rubber tubes, when wastewater circulates in its interior  the wastewater is heated from 4-7 ° C to 15-16 ° C, before entering the ARP. The heat exchanger is continuously heated, due to it is installed on the refrigerator condenser.

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Procedure

Sampling and process control:

The control of the anaerobic digestion process has been carried out by evaluating the following control and operation parameters: temperature, pH, volatile fatty acids The volatile fatty acids, alkalinity AT, TCF tampon capacity factor TCF and and hydrogen redox potential Eh.

The temperature is readed directly on a digital microprocessor, which is connected to a sensor located inside of ARP.

The pH is readed into the microprocessor connected to a pH electrode to the influent, effluent and four composite samples from chamber 1 and 2; 3-5; 6-8 and 9-11 respectively.

The volatile fatty acids  VFA, total alkalinity TA and tampon capacity factor TCF are determnados by potentiometric titration with the help of a magnetic stirrer, for the four composite samples referred above.

The oxide reduction potential relative to the hydrogen electrode Eh, is readed into the microprocessor connected to an electrode Eh, for the four composite samples.

The removal of organic matter expressed as chemical oxygen demand COD is determined by the difference between the COD of raw and treated wastewater.

COD is analyzed and determined by the spectrophotometric method of the potassium dichromate for influent and effluent, respectively, from four-day composite samples  preserved under freezing.

Starting up the RAP-100:

Commissioning of theRAP-100, was carried out on 101 days from 03/01/94 until 09/06/94 and has been initiated under the following characteristics (first 13 days):

Operating parameters:

• Mud seed: septic tank sludge.
• Seed sludge concentration in the chambers of ARP: 1 gr. VSS / I.
• Maximum methanogenic activity of the seed sludge (300C) AM: 0.04 gr. COD / g. VSS. day
• Substrate: urban wastewater, extracted from the main sewage collector of Tarija, at peak hours flow and organic load.
• Hydraulic retention time: 48 hr.
• Ascent rate of flow within the ARP: 0.052 ± 0.003 m / hr.
• Residual water flow loaded to the ARP, Q infl: 8.69 ± 0.55 cc / min.
• Concentration of wastewater, COD e: 365 ± 130 mg. COD / I.
• Volumetric organic load, Lv: 0. 18 ± 0.07 gr. COD / I. day.
• COD removal efficiencies ef%: 94.2 ± 1.8%.

Control parameters:

• Operating temperature: 20.61 ± 0.7 °C.
• Influent pH, pHi: 7.45 ± 0.27.
• Effluent pH, pHe: 7.16 ± 0. 1 1.
• pH Critical Chambers (chambers 1 and 2): 6.97 ± 0.11.
• Volatile fatty acids, pilot control chambers (1 and 2), AVG: 28.3 ± 4.4 mg. of acetic acid /l.
• Total alkalinity, pilot control chambers (1 and 2), TA: 229 ± 41 mg. of  calcium carbonate / 1.
• Tampon capacity factor control chambers (chambers 1 and2) FCT: 0.1 ±0. 01.

After 13 days of operation and due to the high removal efficiencies achieved it was decided to reduce the hydraulic retention time from 48 to 24 hr with rise speeds of 0.1041 m/h. Under these conditions the granulation has been reached (bacterial immobilization in the form of flocs) at approximately 47 days. But despite having achieved partial granulation, is not been considered that the ARP was fully mature until proven its stability for lower hydraulic retention times and under critical temperature. Taking into account the Orozco’s recommendations  – 1994 operating at speeds of 0.25 m / h for greater granulation, it was decided to operate at 0.204 m/h with hydraulic retention time of 12 hr, after 61 days of operation. The stability of the process has finished checking at 101 days approximately with the following operational and environmental conditions in the last 40 days of the launch.

Operation parameters:

• Type of sludge: granular sludge.
• Sludge concentration: 2.76 gr. VSS/1.
• Maximum methanogenic activity of the sludge (300C) AM: 1.01 gr. COD / g. VSS. day.
• Substrate: urban wastewater, extracted from the main sewage collector of Tarija, at peak hours of flow and organic load.
• Hydraulic retention time: 12 h.
• Ascent rate of flow within the ARP: 0.208 ± 0.026 m / h.
• Residual water flow loaded to the ARP Q ini]: 34.68 ± 4.62 cc / min.
• COD concentration of wastewater e 457 ± 96 mg. COD / I.
• Volumetric organic load Lv: 0. 91 + -0.19 g. COD / I. day
• COD removal efficiencies ef%: 87.1 ± 3.4%

Control parameters:

• Operating temperature: 16.81 ± 1.9 ° C.
• influent pH, pHi: 7.86 ± 0.38.
• effluent pH, pHe: 7.45 ± 0.2.
• pH pilot control chambers ( 1 and 2): 7.27 ± 0.29.
• Volatile fatty acids, pilot control chambers (l and 2) VFA : 48.4 ± 16.3 mg. of acetic acid/I.
• Total alkalinity, pilot control chambers (l and 2) TA : 300 ± 66 mg. of calcium carbonate/1.
• Tampon capacity Factor, control chambers (chambers 1 and 2) TCF : 0. 14 ± 0.05.

It should be emphasized that under these conditions the mud occupied 10% of the reactor volume.

Physical Simulation of the daily profile of wastewater flow:

A real-scale treatment plant, is subjected to time variations of wastewater inflow, so it was considered important to study the response of the ARP-100, with maximum and minimum flow peaks, for this has been carried out the design of a feeding flow profile similar to the relative feeding flow profile of the City of Tarija, in this particular case, the wastewater flow has two peaks that occur between 8 and 10 am and from 14-16 hours. By Performing a relative flow analysis, assigning 100 to the maximum flow, it has been found the daily flow profile simulated characteristic of Tarija.

The wastewater flow profile design was performed using iterative calculations, in a spreadsheet has been designed a feeding flow profile to the ARP at laboratory level similar to the profile (see Table 1 and Fig. 3).

By the simulation of the real profile, the average flow fed to the RAP-100 is 52 ± 19 cc / mim with an average hydraulic retention time of 8 hr and an ascend speed of  0.313 ± 0.115 m / h. The ARP was operated under these conditions for 51 days, obtaining important results of removal and stability of the digestion process (see table 2).

Determination of emetic constants of COD removal and NMPCF:

Because the kinetic constants are fundamental data to the design and dimensioning of a biological wastewater treatment reactor, we have proceeded to evaluate them, for the following kinetic models:

The kinetic constants of COD removal have been evaluated for the piston flow kinetic model of Orozco 1989 for ARP- 100:

CODo and CODf [mg. COD / I]: chemical oxygen demand of the influent and effluent of the ARP-100.

X [mg. VSS / L]: concentration of granular sludge.

HRT: hydraulic retention time.

The kinetic constants of pathogen removal expressed as NMPCF, have been evaluated for the kinetic model Polpraset and Hoang – 1983 cited by Belli F-1984:

Ka of removal [1 / day]: kinetic constant of of mortality for fecal coliforms under anaerobic conditions.

NMPCFo and NMPCF [# / 100 ml] most probable number of fecal coliforms for the influent and effluent of the ARP-100.

HRT: hydraulic retention time of.

With the following methodology:

• Determination of COD and NMPCF in the input and output flows of the ARP- 100 for two conditions of hydraulic retention time of (8 and 12 hours), and different operating conditions and controlled anaerobic digestion.
• Kinetic constants calculation.

For the ARP-100 is a hydride reactor inflow piston, according Orozco-1989 and Levenspiel-1985, so we can say for practical purposes that the fluid dispersion factor is zero, according to Forero-1985, on this basis the  hydraulic retention time can be assimilated to the fluid stop time.

Determining the sludge production rate:

The sludge production rate was determined by:

T1 =        X₂ – X₁         

         % EF COD_. L_V.t

where:

X1 [gr.VSS / 1]: sludge concentration at the beginning of the operation period.

X2 [gr. VSS / 1]: sludge concentration at the end of the operation period.

Ef% COD: COD removal efficiencies.

Lv [gr. COD / 1. Day 1: volumetric organic load.

t [day]; period length.

A correlation between the mud development and its methanogenic activity within the ARP is presented in Figure 4.

Determination of pilot control chambers:

For this purpose we have proceeded to compare the averages values, various control parameters evaluated in the following combinations of : 1- 2; 3-5, 6-8 and 9 -11.

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Results and Discussion

By controlled conditions, has been achieved bacterial imobilization through the development of granular sludge with high methanogenic activity, fundamental feature of the ARP concept, according to Orozco -1994.

It has been proven that the septic tank sludge (existing in our environment), in a concentration of 1 gr. VSS / I, is a high-quality inoculum, obtaining partial granulation at 47 days after the start of the ARP-100 although references exists that the seed sludge is not necessary.

There is information about the behavior of the anaerobic digestion process during the commissioning of the ARP, growth of mud and increase of methanogenic activity in the form of chronological tables and profiles of the following control parameters: pH, VFG, TCF, Eh. We have determined the following information for the design and operation of the ARP- 100 for sub-optimal temperatures and subjected a real daily flow profile  during 51 days:

Operation parameters:

• Type of sludge: granular sludge.
• Initial concentration of the sludge in the the ARP: 2.76 gr. VSS/1.
• Maximum initial methanogenic activity of sludge (30 ° C) AM: 1.01 gr. COD / g. VSS. day.
• Final concentration of the sludge in the the ARP: 6.27 gr. VSS/I.
• Sludge formation rate: 0.0592 gr. VSS / gr. of COD removed.
• Final maximum methanogenic activity of sludge (30 ° C) AM : 0.91 gr. COD / g. VSS. day.
• Sludge formation rate: 0.0592 gr. VSS / gr. of COD removed.
• Substrate: urban wastewater, extracted from the main sewage collector of Tarija, at peak hours of flow and organic load.
• Hydraulic retention time: 8 hr.
• Ascent speed of flow within the the ARP: 0.313 ± 0.115 m / h.
• Sewage flow ARP- 100 Q infl: 52.08 ± 19. 1 cc / min.
• Concentration of wastewater CODO: 499 ± 92 mg. COD / I.
• Volumetric organic load Lv: 1.5 ± 0.28 gr. COD / I. day.
• COD removal efficiencies: 77.5 ± 5.5%.
• Removal efficiencies DB05: 84 ± 1.5%.
• Removal efficiencies NMPCF: 67 ± 19%.
• Operating temperature: 13.14 ± 1.8 ° C.

Control parameters:

• influent pH, pHi 7.91 ± 0.43.
• effluent pH, pHe: 7.25 ± 0.2.
• 7.47 ± 0.27 pH pilot control chambers (l and 2) cameras.
• Volatile fatty acids VFA pilot control chambers (1y2): 57.8 ± 9.74 mg. of acetic acid /l.
• Total alkalinity AT, pilot control chambers (l and 2) : 357 ± 56 mg. of calcium carbonate / 1.
• Tampon capacity factor TCF, pilot control chambers (l and 2) : 0.14 ± 0.03.
• Redox potential Eh, pilot control chambers (l and 2) : -143 ± 16 [mv].

Removal efficiencies and control parameters were determined from composite samples  collected for the period during which it was fed maximum flow peaks to the the ARP, from 9:00 am to 9:00 pm, with hydraulic retention time from 6 hr.

Kinetic constants:

For COD removal by Orozco’s model at 14.7 ° C:

K = 0.575 [mg. COD / mg. VSS. day].

K2 = 0.094 [mg. COD / mg. VSS].

The emetics constants determined, are consistent with the values reported by Orozco and the results obtained in the tests from methanogenic activity.

For the NMPCF removal:

Ka = 5.2 ± 2.8 [I / day].

The constant for removal of fecal coliforms Ka, five times larger than that reported by Polpraset and Hoang value quoted by Belli F-1984, probably because the ARP-100, best fits the fluid flow piston model than the anaerobic systems studied by these authors (UASB, septic tanks, etc.).

Sludge production:

The sludge production rate T1 estimated for the last 70 operation days of  the ARP has a value of:

TI = 0.043 gr VSS. of sludge / gr COD removed.

This result confirms the low reproductive rates of anaerobic microbial population within the reactor, which is a great advantage over conventional treatment for activated slugdes.

control pilot cameras:

The stability of the fermentation process based on the control parameters listed in the following table (51 days of operation):

We can affirm that chambers 1 and 2 present the relatively critical values of control parameters considering that the process stability is inversely proportional to the values of Eh, VFG and TCF.

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Conclusions

• The hydraulic retention time of 8 hr seems to be the optimum for scaling, based on the average wastewater flow, taking as the main criteria the removal of organic matter, with %ef DQQ%: 77.5 ± 5.5% efDB05: 84 ± 1.5 and concentrations 38 ± 2.6 gr. DB05/1 in the effluent, for temperatures of 13.1 ± 1.8 ° C.
• The granulation (bacterial immobilization by flocs) has developed ascend speeds of 0.1 to 0.2 m/h.
• The following equation, based on Orozco’s kinetic model of describes the behavior of the ARP, with respect to the COD removal at medium temperatures of 14.7 ± 1.3 ° C:

0.094.X.TRH = 0.575.X.Ln (COD °) / (DQOf) + (COD ° – DQPf)

• The following equation, based on the kinetic model of Polpraset and Hoang, describes the behavior of the ARP, with respect to the removal of conforming bacteria to an average temperature of 14.7 ± 1.3 ° C:
• The sludge production rate is: 0043 gr VSS. sludge / gr. COD removed.
• The chambers 1 and 2, can be used to control the reactor fermentation process.

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Bibliography

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Barnes D. y P.A. Fitzgerald,”Anaerobic Wastewater Treatment Processes” Enviromental Biotechnology ” School of Civil Engineering, University of New Sout Wales Australia 1987.

Femandez F. P.y V. Diez ” Digestión Anaerobia” Tomo III; “Lechos Fluidizados”Departamento de Ingeniería Química Universidad de Valladolid; España., 1989”.

Garcia P.A. y F. Fdz Polanco ” Digestión Anaerobia” Tomo II; ” Filtro anaerobio” Departamento de Ingeniería Química Universidad de universidad de Valladolid; España., 1989″.

Lettinga G., A.F.M. Van Velsen, S.W. Hobma ” Biotecnology and Bioengineering, Vol xxii, Pp 699- 734, 1980, The Netherland, Holanda.

Lettinga y col.” High-Rate Anaerobia Waste-Water Treatment Using the UASB Reactor under a Wide Range of Temperture Conditions.” Biotecnology and Genetic Enginering Revlews- Vol. 2,Oct., 1984.

Lema M. J. y col “Bases Cinéticas y Microbiológicas en el diseño de digestores anaerobios” , Rev. Ingeniería Química, Enero 1992, Univ. Santiago de Compostela- España”.

Lettinga G. y Look Hulshoff ” Advance Reactor Desing, Operatión and Economy. Dep. of Water.

Pollution Control Agricultural Universlty De Dreijen 12, 6703 BC Wagenginen The Netherland, Holanda, 1986″.

Look Hulshoff Pol, Gatze Lettinga y Jin Field ” Digestión Anaerobia” Tomo 1; ” Reactores UASB Universidad Agrícola de Wageningen, Holanda.V; 1989.

Levenspiel Octave, ” Ingeniería de las Reacciones Químicas “, 1985, Otter Rock Oregon, USA.

Muñoz J. A. V. ” Depuración Anaerobia de Aguas Residuales” Rev Alimentación, Equipos, y Tecnologia ” May., Jul., Ago., 1990, Madrid España”.

Molina 0. E. y Nora Perotti ” Apunte sobre Microbilogía” Jul. 1990 Tucuman-RR.AA.

Mercalf & Eddy INC. Biológical Unit. Process, In Wastewater Enginering: Treatment, disposal, reuner. New York, MacGraw Hill. 1979.

Orozco A. ” Digestión Anaerobia” Tomol; “Seminario Internacional Sobre Digestión Anaerobia, Elementos de Diseño” Universidad de los Andes, Fac. de Ingeniería, Dep. de Ing. civil, Bogotá Colombia 1989.

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Acknowledgements

The authors thank the National Fund for the Environment “FONAMA” / Environmental Account “Initiative for the Americas”, for the financial support provided through the project PROMADE 9A / 04e / 04-02. To Eng. Alvaro Orozco for his valuable comments and suggestions for the development of this work.

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Table 1

Juan Misael Saracho Autonomous University-Faculty of Science and Technology.

Pilot Plant of Microbiological Anaerobic Processes of wastewater purification.

PROMADE / FONAMA Environment Account Initiative for the Americas.

Activity 10: Measurement of design parameters of the ARP-100.

Q (cc / min): Flow of wastewater fed to ARP-100, estimated by: Q (cc / min) -4.57 (54 + Zf) / 2Lcap.
where: Zi: initial height of the feed tank for the one hour period.
Zf: Final height of the feed tank for the one hour period.
Lcap: length of the capillary tube of the flow control device.
Vasc (m / h). Fluid ascent speed in the chambers of ARP-100.

Table 2.a

Juan Misael Saracho Autonomous University-Faculty of Science and Technology.

Pilot Plant of Microbiological Anaerobic Processes of wastewater purification.

PROMADE / FONAMA Environment Account Initiative for the Americas.

Activity 10: Measurement of design parameters of the ARP-100.

Summary of the operation of the anaerobic reactor with piston ARP-100. During the simulation of the flow profile for Domestic Wastewater for hydraulic retention time of 6 and 12 hours, with an average of 8 hours.