7 th International Conference on Sewer Processes and Networks (SPN7) Isaac Volschan Jr. Water Resources and Environmental Engineering Department Federal University of Rio de Janeiro Brazilian separate sewerage systems expansion faces the challenge of new alternative engineering solutions: how to deal with grit contribuitio from storm sewers ?

7 th International Conference on Sewer Processes and Networks (SPN7) Population Attendance

7 th International Conference on Sewer Processes and Networks (SPN7) Summer season Winter season 30 days acum Max 1 hour Rio de Janeiro rainfall regime  large difference between summer wet period and winter dry period  30 days accumulation rainfall during wet periods achieves values 3.5 x higher than dry periods  during summer wet periods, 1 hour rainfall event reaches up to 50-80% of 30 days accumulation rainfall  very high rainfall 1 hour events are not frequent

7 th International Conference on Sewer Processes and Networks (SPN7) COMBINED  large structures would be required  be optimized for dry, but also for very high wet flows  present low frequency use, just for few high rainfall events

7 th International Conference on Sewer Processes and Networks (SPN7) STORM SEWERAGE Brazil applies separate system Partially treated = pollution

7 th International Conference on Sewer Processes and Networks (SPN7) Deficient separate systems pollute Irregular dwelling connections & interconnections Irregular housing = irregular connection Decentralized bad performanceFirst flush water quality

7 th International Conference on Sewer Processes and Networks (SPN7) How to improve separate efficiency ? IF SEPARATE AREN’T EFFECTIVEIF COMBINED AREN’T REASONABLE WET DRY WET DRY DRY-WEATHER FLOW WATER INTAKE 1

7 th International Conference on Sewer Processes and Networks (SPN7) How to improve separate efficiency ? IF SEPARATE AREN’T EFFECTIVEIF COMBINED AREN’T REASONABLE DRY-WEATHER FLOW WATER INTAKE 2

7 th International Conference on Sewer Processes and Networks (SPN7) 852 L/s 218 L/s 860 L/s 5.0 m³/s It will improve separate efficiency, but concerns about GRIT … 18.0 x m 3 grit/m 3 (WEF, 1992): t grit/d

7 th International Conference on Sewer Processes and Networks (SPN7) CONCERNS ABOUT GRIT – 1 GRIT DEPOSITION IN PUMP SUMPS  Interruption of pump operation  Material accumulation and even equipment grounding  Inside equipment devices or inside suction and discharge pipes  Higher hydraulic headlosses and even in clogging  Bad odours may occur  Designed to optimize geometrical sump volume and pumping efficiency  Grit removal mechanical equipments must be part of the enginnering solution in order to avoid excess grit accumulation

7 th International Conference on Sewer Processes and Networks (SPN7) CONCERNS ABOUT GRIT – 2 GRIT ABRASIVE INJURIES  Potentially abrasive due to the large amount of silica and its high hardness  Progressive loss of material, loss of equipment efficiency, and operational disruption  Pump impeller vanes, suction entrance devices and the initial section of discharge pipes  Ceramic coating on the pump metal surface, especially at the volute and impeller.  Steel mechanical parts with high chromium content and much higher abrasion resistance

7 th International Conference on Sewer Processes and Networks (SPN7) CONCERNS ABOUT GRIT – 3 GRIT DEPOSITION IN PIPES Brazilian guidelines design TRACTIVE TENSION  1,0 - 1,5 Pa Avoid deposit of organic solids < 0.2 mm, anaerobic degradation, hydrogen sulfide and odors Gupta, 2008 Tractive tension requirements Particle diameter d 50 (mm) Without thin sediments Low quantity thin sediments High quantity thin sediments  frequent cleaning and unclogging should be required  main pipe: dia 2,000 mm and 13 m deep  mainholes are up 300 m apart  difficult and costly operation

7 th International Conference on Sewer Processes and Networks (SPN7) CONCERNS ABOUT GRIT – 4 GRIT DEPOSITION AT WWTP  To prevent sedimentation in primary clarifiers  Alegria WWTP: 5 grit chambers, square type  3 years operational data: 90t grit/month  1.5 t/m³ = 60 m³/month  Q=1.38 m³/s (36.3x10 5 m³/m): 1.65x10 -5 m³ grit/m³  Q=5.0 m³/s (129 x 10 5 m 3 /month): 214 m³ of grit/m just due to sewage from separate sewerage network  Q DWWI total flow= 50 x 10 5 m³/month  18.0 x m 3 of grit/m 3 : 900 m³ of grit/m  4 times the regular removal at Alegria WWTP  Wouldn’t disturb mineral discrete sedimentation  Would affect scrapers operation and effective removal  Sedimentation in primary clarifiers  Anaerobic Digestors may soon become overloaded.

7 th International Conference on Sewer Processes and Networks (SPN7) CONCERNS ABOUT GRIT – 5 URBAN AND ENVIRONMENTAL IMPACTS  Negative visual aspects at important sites of the city  Vehicles for operation and maintenance services, in special for removal of grit, may impact local traffic  Risk to odour generation.

7 th International Conference on Sewer Processes and Networks (SPN7) How to improve separate efficiency ? IF SEPARATE AREN’T EFFECTIVEIF COMBINED AREN’T REASONABLE IN RIVER WATER TREATMENT ?