Keywords anaerobic reactor effluent; dissolved-air flotation; domestic sewage; phosphorus removal; post-treatment; flocculation

Transcription

1 Influence of flocculation conditions in the performance of an experimental domestic sewage treatment plant consisting of an anaerobic expanded bed reactor followed by dissolved air flotation R. G. Penetra; M. A. P. Reali & J. R. Campos Department of Hidraulic and Sanitary Engineering, São Carlos School of Engineering, University of São Paulo (SHS-EESC-USP), Av. Trabalhador São-carlense,, São Carlos - SP - Brasil, ZIP code: -, penetra@otv.com.br; mapreali@sc.usp.br Abstract This paper presents the results of a study performed with an experimental domestic sewage treatment plant ( m.d - flow) consisting of expanded bed anaerobic reactor (EBAR) followed by dissolved air flotation (DAF) unit. For flotation step, the anaerobic reactor effluent was previously coagulated with mgfecl.l - and flocculated under different conditions (mean velocity gradient, Gf, and flocculation time, Tf). The Gf values were from to s - associated with and min Tf values. During the tests, the following operational conditions of the flotation unit were maintained: chemical addition ( mgfecl.l - ), % recirculation rate associated with a pressure of ± kpa in the saturation chamber and overflow rate of m.m -.d -. Temperature ranged from.ºc to.ºc. Best results were achieved for Gf= s - and Tf= min. For these conditions, the DAF unit removal efficiencies were:.% for chemical oxigen demand (with mg.l - COD residual), % for phosphorus (with. mgp.l - residual),.% for total suspended solids (with mg.l - TSS residual) and.% for turbidity (with. NTU residual), when the anaerobic reactor effluents have worst quality during the whole day. Keywords anaerobic reactor effluent; dissolved-air flotation; domestic sewage; phosphorus removal; post-treatment; flocculation Introduction In hot climate countries, like Brazil, the use of anaerobic wastewater treatment is becoming more and more frequent. Among the anaerobic reactors, the expanded bed anaerobic reactor (EBAR) has a great technological development potential due to its special design and operational feature. According to Campos & Pereira (), generally, the anaerobic reactors nowadays present satisfactory performance, however, considering their hydrodynamics, they still don t present optimized conception. The EBAR would be the one among the anaerobic reactors that may offer better hydrodynamic conditions. Due to this feature, this kind of reactor needs lower hydraulic detention times, if compared with another known anaerobic reactors. In EBAR the microorganisms grow attached on the support bed, kept suspended by the ascending movement of the liquid. The rising velocities used in EBAR are much higher due to a bigger density of its bio-particles (support material + bio-film) in comparison with those present in UASB - Upflow Anaerobic Sludge Blanket - reactors (biological granules). However, hardly their effluents qualities obey the current Legislation. Then, a post-treatment system for organic matter, suspended solids and nutrients additional removal is frequently needed. Dissolved air flotation units have excellent features, since dealing with high rate processes with excellent removal of suspended solids and flocs produced in chemical coagulation of effluents (Bratby, and Odegaard, ). From on, a staff group from São Carlos Engineering School/USP/Brazil started research aiming the use of dissolved air flotation (DAF) as post-treatment of pilot-scale UASB reactors (Campos et. al., ; Penetra et. al., ; Reali et. al., a and Reali et. al., b). These studies indicated that the anaerobic reactor and dissolved air floater could compound system able to remove about % COD, % phosphate,,% suspended solids,,% turbidity and % of apparent color, using mg.l - of ferric chloride. Recently, following the same research line, a wastewater treatment system made up of an EBAR followed by a dissolved air flotation unit was

2 constructed in a large pilot-scale, treating m.d - of wastewater coming from the public sewage of São Carlos/SP/Brazil. This paper presents the results obtained from this plant, adding mg.l - of ferric chloride for coagulation of the anaerobic effluents and applying different mean gradient velocities (Gf) and flocculation time (Tf) before flotation. Methods The experimental wastewater treatment plant used for this study is composed of an expanded bed anaerobic reactor (EBAR) followed by a dissolved air flotation unit (DAF) fed with a flow average of m.d - of sanitary sewage (see Figure ). The. m tall and. m wide EBAR has a reaction zone followed by a sedimentation zone near the outlet (. m wide) resulting in a total volume of m. This reactor has a recirculation pumping system to make the control of the upflow sewage velocity into it possible. A B Figure Expanded bed anaerobic reactor (A),. m tall, and dissolved-air flotation unit (B),. m tall. EBAR effluent passes through an in-line rapid mix unit just before entering the DAF system, where the ferric chloride solution was applied. In the first part of the DAF system, there are four square flocculation chambers (volume of. m ) having independent slow mixers controlled by frequency inverters. Together with the flocculation zone there is a rectangular (. m long,. m wide and. m tall) DAF unit, having an automatic surface scrapper to remove the floated sludge. The recirculation system of DAF unit has a packed saturation chamber (. m wide and. m tall) that worked under ± kpa during the experiments. Most part of DAF system is automatic and the flows (influent and recirculation) are monitored by inductive electromagnetic flow meters. During the experiment, two mean flocculation times (Tf) were tested: min and min. For each Tf, the following mean velocity gradients (Gf) values were investigated: s -, s - and s -. During the essays the following operational conditions were maintained: ferric chloride dosage (FCD): ± mg.l - ; recirculation flow: ± m.d - ; gauge pressure in the saturation chamber: ± kpa. All these conditions were adopted based on the results from a previous research of Penetra et. al. () and Reali et. al. (). The overflow rate applied on the DAF unit was fixed at ± m.m -.d -. Table shows the main operational conditions of the essays. Each essay was performed from AM (:) to PM (:), and the samples were collected each three hours interval. Raw sewage samples (after passing through strains and sand removal), EBAR effluent and DAF effluent were collected. The following parameters were determined: ph, turbidity, temperature, COD, phosphorous and total suspended solids (TSS). These analyses were carried out according to Standard Methods for the Examination of Water and Wastewater ().

3 Table Schedule of the Experimental Program Essays FCD mg.l - Tf min Gf s - Recirculation rate (%) Saturation Pressure (kpa) Flotation rate (m.m -.d - ) ± ± ± ± ± ± Results and discussion Turbidity Removal: The raw sewage turbidity values varied during the essays from NTU to NTU. EBAR effluent presented turbidity values between and NTU, as seen in Table. Considering all samples collected throughout the essay, EBAR kept mean turbidity removal efficiencies in the range of % and %. Using min Tf associated to s - (essay ), an effluent was produced with turbidity between to NTU and mean turbidity removal was.%. Considering the EBAR-DAF system, with the same conditions of essay, a global turbidity removal of.% was obtained. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a significant mean turbidity removal improvement (.%) by the DAF unit, with values between. and. NTU. Consequently, the total system removal was also increased to.%. Using mean Gf of s - (essay ) provided little turbidity removal improvement (.%), in comparison to the values related to s - Gf. The values at the final effluent of essay ranged from. to. NTU. The total removal practically did not varied using Gf of s - (.%) and s - (.%). Table Turbidity and COD values from different samples collected in the essays. Turbidity (NTU) COD (mgo.l - ) Tf = min Tf = min Tf = min Tf = min Time Samples s - s - s - s - Sewage : EBAR effluent.. DAF effluent Sewage : EBAR effluent.. DAF effluent Sewage : EBAR effluent. DAF effluent Sewage : EBAR effluent DAF effluent Sewage : EBAR effluent DAF effluent When min Tf was applied associated to s - Gf (essay ), the turbidity of DAF effluent ranged from. to. NTU and mean removal was.%. Considering the EBAR-DAF system, with the same conditions of essay, a whole turbidity removal of.% was reached. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a great mean turbidity removal improvement (.%), by the DAF unit; in this case values between. and. NTU were obtained. This way, the whole removal efficiency of the system (EBAR-DAF) was also increased to s - s - s - s - s - s - s - s -

4 %. Using mean Gf of s - (essay ) resulted in a decrease of turbidity removal (%), in comparison to when s - Gf was applied. The turbidity value of the final effluent of essay varied from. to. NTU. The total system s removal presented a slight decrease when using s - Gf (.%) in comparison to s - (.%). Increasing the flocculation time from to min provided a great increase in turbidity removal efficiency, from. to.%, when around s - Gf was used. For s - Gf, increasing the flocculation time from to min provided a significant increase in turbidity removal efficiency, from. to.%. A small increase, from. to.%, was seen when the flocculation time was at min and s - Gf was used. Considering the total removal, there were practically no meaningful turbidity removal efficiencies differences in using both Tf ( and min) and Gf ( and s - ), with values between. and.%. Organic Removal: The raw sewage COD values varied between and mg.l -. EBAR effluent presented COD values in the range of and mg.l -, as seen in Table. Considering the different samples collected throughout the essays, the EBAR kept organic load removal efficiencies (expressed in COD) around and %. Using min Tf associated to s - Gf (essay ) produced an effluent with COD between and mg.l - and mean organic load removal of.%. Considering the EBAR-DAF system, with the same conditions of essay, the total organic load removal was.%. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a great mean organic load removal improvement (.%) by DAF, with COD values between and mg.l -. Consequently, the total removal of the system was also increased to.%. The use of s - Gf (essay ) provided a slight improvement in organic removal, around.%, in comparison with the values related to s - Gf. The COD value in the final effluent of essay varied between and mg.l -. The total COD removal varied only slightly among the operational conditions of s - (.%) and s - (.%) Gf. Using min Tf associated to s - Gf (essay ) produced an effluent with COD between and and mg.l - and mean organic load removal of.%. A whole organic load removal of.% was seen in the EBAR-DAF system, with the same conditions of essay. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a significant mean organic load removal improvement (.%) by DAF, with COD values between and mg.l -. Consequently, the whole removal was also increased to.%. The use of s - Gf (essay ) provided a small organic load removal decrease, around.%, in comparison with the values related to s - Gf. The final effluent COD values of essay varied between and mg.l -. The total COD removal presented a slight decrease when using s - Gf (.%) than when s - Gf (.%) was applied. Increasing the flocculation time from to min (Figures A and B) provided an increase in organic load removal efficiency, from. to.%, when s - Gf was used. For mean Gf of s -, increasing flocculation time from to min provided a slight increase in organic load removal efficiency, from. to.%. A small reduction, from. to.%, was seen when the flocculation time was increased from to min with s - Gf. Considering the whole removals, there were practically no organic load removal differences using Tf of and min and Gf of and s - combined (values between. and.%). Phosphorous Removal: The total phosphorous content ranged from. to. mg.l -. The EBAR effluent presented phosphorous content between. and. mg.l -. Considering the different samples collected throughout the essays, the EBAR kept phosphorous removal efficiencies between. and.%. Using min TF associated to s - Gf (essay, Figure A, Table ) produced an effluent containing phosphorous content between. and. mg.l - and mean phosphorous removal of.% in the DAF unit. Considering the EBAR-DAF system, with the same conditions of essay, a whole phosphorous removal of.% was reached. Keeping the same flocculation time and

5 increasing Gf to s - (essay, Figure C) resulted in a great mean phosphorous removal improvement (.%) by the DAF unit, with P values between. and. mg.l -. Consequently the total removal was also increased to.%. Using s - Gf (essay, Figure E) resulted in phosphorous removal of.%, quite similar to the removal when s - Gf was applied. The final effluent P values of essay varied between. and. mg.l -. The whole phosphorous removal presented a slight decrease when s - Gf was used (.%), in comparison to s - Gf (.%). A COD (mg.l - ) C COD (mg.l - ) E COD (mg.l - ) Tf = min and Gf = s - Sewage EBAR effluent DAF effluent Tf = min and Gf = s - Tf = min and Gf = s - COD (mg.l - ) COD (mg.l - ) Tf = min and Gf = s - Figure Chemical oxygen demand (COD) in the effluents (sewage, EBAR and DAF) in essays (A), (C), (E), (B), (D) e (F). B D F COD (mg.l - ) Tf = min and Gf = s - Tf = min and Gf = s - Using min Tf together with s - Gf (essay, Figure B) produced an effluent with P between. and. mg.l - and mean phosphorous load removal of.%. The EBAR-DAF system, with the same conditions of essay, produced phosphorous removal of.%. Keeping the same flocculation time and increasing Gf to s - (essay, Figure D) resulted in a great mean phosphorous removal improvement (.%) by the DAF unit, with P values between. and. mg.l -. Therefore, the whole removal was increased to.%. Using s - Gf (essay, Figure F) provided a small mean phosphorous removal decrease (.%), in comparison to s - Gf. The P values in the final effluent of essay varied in the range of. and. mg.l -. The whole removal presented a slight decrease when using s - Gf (.%) than when using s - Gf (.%)

6 A P (mg.l - ) C P (mg.l - ) E P (mg.l - ) Tf = min and Gf = s - Tf = min and Gf = s - Tf = min and Gf = s - Sewage EBAR effluent DAF effluent B P (mg.l - ) D P (mg.l - ) F P (mg.l - ) Tf = min and Gf = s - Tf = min and Gf = s - Tf = min and Gf = s - Figure Phosphorus content in the effluents (sewage, EBAR and DAF) in essays (A), (C), (E), (B), (D) e (F). Raising the flocculation time from to min provided a small phosphorous removal increase, from. to.%, when s - Gf was used. For s - Gf, raising the flocculation time from to min also provided only a small organic removal efficiency increase, from. to.%. Quite similar removals (. and.%) were seen when the flocculation time was raised from to min and s - Gf was used. Considering the whole removal there were practically no meaningful differences in phosphorous removal using combined flocculation times of and min and and s - Gf, with values between. and.%. Apparently, using s - Gf provided small variations in phosphorous content in the DAF unit effluent in samples collected in different schedules throughout the day. In the essay using and s - Gf (Figures A, B, E and F), peaks of phosphorous content occurred in samples at PM (:), while in essays with s - (Figures C and D) the phosphorus contents were more stable. Suspended Solids Removal: The TSS raw wastewater values during the essays ranged from to mg.l -. The EBAR effluent presented TSS values between and mg.l -. Considering the

7 different samples collected throughout the essay, the EBAR promoted suspended solids load removal efficiencies between. and.%, as shown in Table. Using min of flocculation time along with s - Gf (essay ) produced and effluent with TSS content between and mg.l - and mean removal of.%. Considering the EBAR-DAF system, with the same conditions of essay, a total TSS removal of.% was observed. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a great mean TSS removal improvement (.%) by the DAF unit, with TSS values between and mg.l -. This way, the whole removal was also increased to.%. Using s - Gf (essay ) provided TSS removal, around.%, similar to the one seen when s - was used. TSS values in the final effluent of essay varied between and mg.l -. The whole TSS removal presented a slight decrease when s - was used (.%) than when s - was tested (.%). Table Phosphorus content and total suspended solids in different samples collected in the essays. Time Sample s - Phosphorus (mgp.l - ) TSS (mg.l - ) Tf = min Tf = min Tf = min Tf = min s - s - s - Sewage : EBAR effluent DAF effluent Sewage : EBAR effluent DAF effluent Sewage : EBAR effluent DAF effluent Sewage : EBAR effluent DAF effluent Sewage : EBAR effluent DAF effluent s - s - s - s - s - s - s - s - Using min of flocculation time together with s - Gf (essay ) produced an effluent with TSS between and mg.l - and mean TSS removal of.%. Considering the EBAR-DAF system, with the same condition of essay, a whole TSS removal of.% was obtained. Keeping the same flocculation time and increasing Gf to s - (essay ) resulted in a great mean TSS removal improvement (.%) by DAF, with TSS values between and mg.l -. Consequently the whole removal also increased to.%. Using s - Gf (essay ) provided a mean TSS removal reduction, around.%, in comparison to s - Gf. TSS values in the final effluent of essay varied between and mg.l -. The whole removal presented a small decrease when using s - Gf (.%) instead of s - Gf (.%). Increasing the flocculation time from to min provided a mean TSS removal efficiency increase, from. to.%, when around s - Gf was used. For Gf around s -, increasing the flocculation time from to min also provided a high TSS removal increase, from. to.%. In the same way, when s - Gf was used, there was also a substantial TSS removal increase, from. to.%. Apparently, increasing the flocculation time from to min presented pretty positive aspects regarding TSS removal, regardless of the Gf used. Particularly, the use of s - Gf resulted in lower and more stable TSS values throughout the DAF unit operation.

8 Conclusions Generally, the flotation system after EBAR, when operated with adequate coagulation/flocculation conditions, presented easy handling, stable behavior and excellent performance due to great floatability characteristics of the flocs formed in chemical coagulation of EBAR effluent. For flocculation times between and min, the Gf values showed to great influence the flotation efficiency in this kind of effluent. Applying s - Gf provided a smaller TSS values variation in the DAF unit effluent. Particularly, applying s - Gf and min Tf resulted in smaller and more stable TSS values in the EBAR-DAF effluent, throughout the DAF unit operation. Considering the EBAR-DAF total removal, there were practically no meaningful differences in organic load removal efficiencies (. to.%) applying the combined flocculation times of and min and Gf of and s -. Using s - Gf provided a smaller phosphorous content variation in the DAF effluent. At the best mean operational conditions (Gf= s - ; Tf= min; P= kpa; recirculation flow equal to % of the affluent flow and mg.l - of ferric chloride), the EBAR-DAF system produced an effluent with the following residual contents: mg.l - COD (.% removal),. mg.l - phosphorous (.% removal), mg.l - TSS (.% removal) and. NTU (.% removal). These data correspond to the moment when the raw sewage presented its highest contaminants contents. Acknowledgement The authors gratefully acknowledge the financial support to this research effort provided by: FAPESP Fundação de Amparo a Pesquisa do Estado de São Paulo, CNPq Conselho Nacional de Desenvolvimento Científico e Tecnológico and FINEP Financiadora de Estudos de Projetos, from Brazil. References Bratby, J.R. (). Treatment of raw wastewater overflows by dissolved-air flotation. Journal WPCF, (),-. Campos, J. R.; Reali, M.A.P.; Dombroski, S.A.G.; Marchetto, M. & Lima, M.R.A. (). Physico-chemical treatment of anaerobic reactor effluents by flotation. XXV Congreso Interamericano Ingeniería Sanitaria y Ambiental, Mexico. (In Portuguese). Ødegaard, H. (). Optimization of flocculation/flotation in chemical wastewater treatment. Wat. Sci. Tech., (-), -. Penetra, R.G.; Reali, M.A.P.; Foresti, E. & Campos, J.R. (). Post-treatment of effluents from anaerobic reactor treating domestic sewage by dissolved-air flotation. Water Science and Technology, (), -. Reali, M.A.P.; Campos, J.R. & Penetra, R.G. (a). Sewage treatment by anaerobic biological process associated with dissolved air flotation. Water Science and Technology, (), -. Reali, M.A.P.; Penetra, R.G. & Carvalho, M. E. (b). Flotation technique with coagulant and polymer application applied to the post-treatment of effluents from anaerobic reactor treating sewage. Water Science and Technology, (), -. Standard Methods for the Examination of Water and Wastewater (). th edn, American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA.