Impact of Stormwater Management Practices on Wastewater Treatment

Damir Brdjanovic, PhD Professor of Sanitary Engineering

• Traditional Civil Eng. approach vs modern Process Eng approach • Multidisciplinary team

Network • Civil Engineer • Hydraulic engineer • Hydroinformatics • Mechanical engineer • Electrical engineer • Environmental engineer

Plant • Process Engineer/Technologist • Mechanical engineer • Electrical engineer • Civil Engineer • Hydraulic engineer • Architect

PAST, PRESENT and FUTURE

Sitio de tratamientoPlanta San FernandoTratamiento secundario

Sitio de tratamientoTratamiento preliminar

Sitio de tratamientoTratamiento preliminar

Interceptor

Alcantarillado de colección

BARBOSASitio de tratamientoPlanta BelloTratamiento secundario

INTEGRATION!

Separate sewers – household sewage and industrial wastewater by sanitary sewers, urban surface runoff by storm sewers Combined sewers – mixture of sewage/wastewater (DWF) and storm/rainfall runoff (WWF)

Design flowrates

Source: Wastewater Engineering, Metcalf & Eddy, 2002

Typical average composition of untreated/raw domestic wastewater

Source: Wastewater Engineering, Metcalf & Eddy, 2002

Parameter

Sewage strength

Strong Medium Diluted Week

BOD5 350 250 150 100

COD 750 550 350 200

TN 80 50 30 20

TP 25(15)* 15(10)* 10(5)* 5(3)*

SS 450 300 200 100

Coliform bacteria

1012 1012 1012 1012

* When P-free detergents are used

Wastewater principle: Concentrate – DO NOT dilute!

Dilution from

• Infiltration/inflow(CS and SS)

• High qsp (SS)

• Present sanitation practices (5-14 L per flush toilet) (CS and SS)

• Storm water (CS)

Characteristics of wastewater combined with other sources

Source: Wastewater Engineering, Metcalf & Eddy, 2002

from previous slide.

Pollution accumulation and wash off • Dry deposition of atmospheric pollutants on roofs • Deposition of pollutants of traffic on streets, highways and parking places • Accumulation of dust, dirt and larger residues on streets • Solids generation by roadway deterioration • Highway surface contamination by vehicle and tyre wear, fluid spills and leaks • Road surface contamination by salting and sanding in winter conditions

• Wash off by energy of storm generated runoff (wash off and erosion) • Wash off by dissolution and elutriation due to acidity of rainfall

• Leaking septic tanks in non-sewered areas • Illicit discharges into storm sewer systems (car oil, detergents, illegal dumps) • Discharges of grey wastewater and solid waste in surface drainage • Contaminated infiltration • Illegal sanitary sewage connections to storm water system • Build-up of solids in sewers during DWF

First flush •Small systems - more sensitive •Large systems - dilution effect •First15-20 min •First 25 mm 90% of pollution •Effects of street cleaning low (<10% removal)

Urban runoff –

‘first flush’

Capacity 4-8 times DWF CSOs –quantity and frequency regulated and limited Some regulations 90% of the overflow volume stored + treated Storage/retention/detention Buffering/equalization

Stormwater management within the network

• reduce the stormwater volume entering the collection system • optimize the peak flow and related storage capacity of collection system • use off- and in-line surface and subsurface storage strategies for peaks • selectively allow wet weather overflow to capture the first flush

Snow melting

• Water content app. 10% (100 L in 1 m3) • Melting @ above 5ºC, top down • Intensity of melting (temperature, sunlight etc) • Temperature app 0ºC • Problems to WWTP – Q and T

Sediment transport

• Sand in the winter • Land erosion • Illegal dumping • Sand by the wind (beaches, etc) • Surface run-off • Problems to WWTP intake structures

MANAGEMENT PRACTICES

• Network (CSOs & Retentions) • WWTP

CSO

Stormwater detention within the network

Stormwater management at WWTP

• bypassing flows > 6 DWF • use stand by units (pumps, screens, grit chambers) • use of additional plant capacity (for newer under-loaded plants) • shorter settling retention time • increased sludge recirculation

Influent

Pumps and screens

Grit chambers Primary settling tanks

Biology

Secondary settling tanks

Stormwater detention at sewage works

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INLET PUMPING STATION Retention min 5-10 min + storage in the main collector and network (hydraulics and control)

SCREENING Retention <1 min

SEND/GRIT REMOVAL Retention 4 min (WWF) or 12 min (DWF)

PRIMARY SETTLING Retention 1.5-2.5 hrs

BIOLOGY Retention 10-15 hrs

SECONDARY SETTLING Retention 2 hrs

Equalization at WWTP

• Collector • Inlet pump sump • Screens • Grit chambers • Primary clarifiers • Biological tanks • Secondary clarifiers

Flow

Time

Influent

Effluent

• Time lag • Levelling

New developments

• Separation at source • Dry sanitation • Sewerage as bio-reactor (nitrification, fermentation)

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