7th 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 ?

7th International Conference on Sewer Processes and Networks

(SPN7)

Population Attendance

7th 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

7th 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

7th International Conference on Sewer Processes and Networks

(SPN7)

STORM

SEWERAGE

Brazil applies separate system

Partially treated = pollution

7th International Conference on Sewer Processes and Networks

(SPN7)

Deficient separate systems pollute

Irregular dwelling connections & interconnections

Irregular housing = irregular connection

Decentralized bad performance First flush water quality

7th 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

7th International Conference on Sewer Processes and Networks

(SPN7)

How to improve separate efficiency ?

IF SEPARATE AREN’T EFFECTIVEIF COMBINED AREN’T REASONABLEDRY-WEATHER FLOW WATER INTAKE 2

7th International Conference on Sewer Processes and Networks

(SPN7)

852 L/s218 L/s

860 L/s

5.0 m³/s

It will improve separate efficiency, but concerns about

GRIT …

18.0 x 10-5 m3grit/m3 (WEF, 1992): 13.3 + 3.4 + 13.4 t grit/d

7th International Conference on Sewer Processes and Networks

(SPN7)

CONCERNS ABOUT GRIT – 1GRIT 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

7th International Conference on Sewer Processes and Networks

(SPN7)

CONCERNS ABOUT GRIT – 2GRIT 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

7th International Conference on Sewer Processes and Networks

(SPN7)

CONCERNS ABOUT GRIT – 3GRIT 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 d50 (mm)

0.1 0.2 0.5 1.0 2.0 5.0Without thin sediments 1.2 1.3 1.5 2.0 2.9 6.0Low quantity thin sediments

2.4 2.5 2.7 2.9 3.9 8.1

High quantity thin sediments

3.8 3.8 4.1 4.4 5.4 9.0 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

7th International Conference on Sewer Processes and Networks

(SPN7)

CONCERNS ABOUT GRIT – 4GRIT 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 (36.3x105m³/m): 1.65x10-5m³

grit/m³ Q=5.0 m³/s (129 x 105 m3/month): 214 m³ of

grit/m• just due to sewage from separate sewerage

network QDWWI total flow= 50 x 105 m³/month

18.0 x 10-5 m3 of grit/m3: 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.

7th International Conference on Sewer Processes and Networks

(SPN7)

CONCERNS ABOUT GRIT – 5URBAN 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.

7th 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

?