5 Drainage Systems

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City University of Hong Kong Division of Building Science and Technology

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Drainage System

1 Introduction

1.1 Types of Effluents

3 types of effluents according to their states

- solid- liquid and slurry- gas or vapor

1.2 Types of Drainage systemDrainage systems are intended to carry liquid and slurry effluents from buildings andconvey them to suitable locations for discharge or treatment. Types of drainagesystems in Hong Kong can be classified as:- storm water system - a collection of rain water- waste water system - a collection of wastes from laundries, from sanitary such as

wash basins, sinks, showers, bath tubs

-

soil and waste water system - a collection of waste water plus human wastes fromwater closets and urinals- kitchen wastes system a collection of effluent from kitchen's sinks and floor

drain

Fig. 1 shows the general arrangements of storm water drainage and sanitary drainage

systems for a building.

Fig. 1 Typical storm & soil and waste system


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1.3 Connection to Government Public Drainage Networks

In Hong Kong, there is a statutory requirement that storm water drainage systemsmust not be mixed with sanitary drainage systems. Public Works Department providesseparate public sewer and drainage systems for them.

Reasons:- storm water is relatively clean and does not require treatment before discharge- soil and waste water need elaborate treatment before discharge- public sewage treatment plant is designed to handle an anticipated amount of grey

and black water- mixing storm water with soil and waste water overload the capacity of public

sewage treatment plant- discharges from the public sewage treatment plant may contain excessive

pollutants to contaminate the environment

2. Storm Water Drainage System

Cater for rain water of roofs and paved areas.

Storm water flows to the drain by gravity.

2.1 Roofs Storm Water Drainage System

2 types of roofs: pitched and flat

2.1.1 Pitched Roofs

Eaves gutters are provided to catch rain running off of the pitched roofs and downpipes are provided to carry water to the ground level as shown in Fig.2.

Fig. 2 Typical eaves detail showing half round gutter,gutter outlet, swan-neck and rain water pipe


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2.1.2 Flat Roofs

Slightly fall on the flat roof is constructed to carry rain water directly to a roofoutlet/channel formed on the roof leading to an outlet or an eaves gutter.

Features:- rain water pipes may run either externally or internally in a building- pipe joints must be water tight to avoid leakage during full bore flow- sound insulation should be provided when rain water pipes are enclosed behind

wallsFig. 3 shows the storm water drainage system for high rise building.

Fig.3 storm water drainage system for a high rise building

2.2 Paved Areas Storm Water Drainage System

Features- paved areas must be laid to fall towards channels or gullies- all paved surfaces over 6m2should be drained and with a minimum fall of 1 in 60

falls should run away from the building- paved areas may discharge to channels in site roads, or specially formed channels

forming part of the paving


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3. Soil and waste Systems

Soil and waste system consists of soil pipe, waste pipe, vent pipes, traps and sanitarysuch as baths, sinks, lavatory basins, floor drains in lavatories urinals and waterclosets.

General requirements of soil and waste system- satisfactory standard of materials and workmanship is required- pipes should be water tight with smooth internal bores- junctions should be tight and without risk of blockage- joints should be properly aligned with no ridges to catch solid matters- all pipes should discharge effluents they receive immediately- all pipes must fall continuously to ensure proper flow- no arches and dips are allowed along the pipe works- drainage system should convey effluents to manholes and from there to terminal

manhole- the terminal manhole should be connected to Government public sewer or to the

private sewage treatment plant- traps and vents should be provided to drainage systems- soil and waste water flow to the drain by gravity

3.1 Traps and Vents

3.1.1 Traps

Function of traps- The major aims of traps are to separate unhealthy gases in a sanitary drainage

system from coming out of the sanitary fittings. It can be achieved by the watercaught in a trap after every discharge from a sanitary fitting

Types and installation methods- traps can be bottle type (Fig. 4a),P-type (Fig. 4b) and S-type (Fig. 4c)- have standard seal with50mm or deep seal with 75mm (Fig. 4d)-

traps are of steel, cast iron, copper, plastic or brass- traps for water closets and urinals are often of vitreous china cast integral with thefixtures

- sanitary fittings can have individual traps or a common trap- traps are usually placed within 600mm of each fixture (individual trap), or of the

nearest fixture to the vertical stack (common trap)- traps should be accessible for cleaning- overflow pipes from fixtures are to be connected to the inlet side of the trapSpecial attention- sanitary fixtures rarely use may have water in the trap being evaporated, thus

breaking the seal- this is common for floor drains

3.1.2 Vents

Functions of vents- allow air from the fresh air inlet to rise through the drainage system and carry

away offensive gas- to break the possible siphonage of water out of the trap- to prevent the bubbling of foul gas through the trap seal


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Fig.4a Bottle type trap Fig.4b P-type trap

Fig.4c S-type trap Fig.4d Depth of Seal

4.0 Types of Soil and Waste Drainage System

4 main types of soil and waste drainage system- 2-pipe system- full ventilated 1-pipe system- modified 1-pipe system- single stack system

4.1 2-Pipe System (Fig. 5)

Drainage pipes- separate soil pipe and waste pipe- soil pipe connects to water closets, urinals, slope sinks and similar soil appliances- waste pipe connects to all other sanitary fixtures and drains through a trapped

gully

Traps- standard traps (50mm) are provided to all fixtures

Vent pipes- soil pipe and waste pipe have its own vent pipes running through the full height of

the building

- all sanitary fixtures are connected to their respective vent pipes


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4.2 Fully Ventilated 1-Pipe System (Fig. 6)

Drainage pipes- soil and waste pipes are combined into one common stack- all sanitary fixtures are connected to the common stack

Traps- deep seal traps (75mm) are provided to baths, basins and sinks- water closets and urinals are provided with integral standard traps (50mm)

Vent pipes- a vent pipe runs through the full height of the building to which all sanitary

fixtures are connected

4.3 Modified 1-Pipe System (Fig. 7)Drainage pipes- soil and waste pipes are combined into one common stack- all sanitary fixtures are connected to the common stack

Traps- deep seal traps (75mm) are provided to baths, basins and sinks- water closets is provided with integral standard traps (50mm)

Vent pipes- a vent pipe runs through the full height of the building to which only water closets

are connected

4.5 Single Stack System (Fig. 8)

Drainage pipes- soil and waste pipes are combined into one common stack

- all sanitary fixtures are connected to the common stack individually and should belocated close to the stack

Traps- deep seal traps (75mm) are provided to all sanitary fixtures with 50mm or below

stack- standard traps (50mm) are provided to all sanitary fixtures with stack larger than

50mm

Vent pipes- no vent pipe be provided

Drainage pipe size

- 90mm for two storey buildings- 100mm for buildings up to 8 storeys- 125mm or 150mm for higher buildings


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Fig.5 2-pipe system


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Fig.6 Fully vented one-pipe system

Fig. 7 Modified 1-Pipe System


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Fig.8 Single stack system

5.0 Miscellaneous Drainage Systems

5.1 Drainage of Basement Floors

Operations- it is a pumped system, while ordinary drainage systems are operated by gravity- Surface water, waste and soil flow by gravity into separate sump tanks- liquids in the sump tanks are then lifted up by sewage pumps to a height at which

they can fall by gravity into the main drainage pipes at ground floor

5.2 Kitchen Waste Drainage System

Operations- it is similar to ordinary system- the waste has to travel through grease interceptors- as waste from kitchen sinks passes through the circuitous path of a grease

interceptor, the grease floats to the top where it is trapped between baffles- other fluid waste passes through at a lower level- grease collected in these interceptors has to be removed frequently- Interceptors for as many as 25 different types of extraneous matters are provided

by manufacturers. They include devices to catch hair, grease, plaster, lubricating

oil, glass grindings, and troublesome unwanted materials from many industrialprocesses.


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6. Sewage Treatment6.1 Introduction

Wastewater collected from communities must ultimately be returned to receivingwater or to the land. The collection of wastewater includes sewage from residentialand commercial facilities, chemical wastewater from industries and livestock wastefrom villages. The control of waste water disposal is executed by the EnvironmentalProtection Department (EPD) under the Water Pollution Control Ordinance and theWaste Disposal (Livestock Waste) Ordinance.

Under the Water Pollution Control Ordinance, water discharge in Hong Kong iscontrolled according to the Technical Memorandum Standards for EffluentsDischarge into Drainage and Sewerage Systems, Inland and Coastal Waters. Thestandards for effluents discharge is classified into the following categories:

- Foul sewers leading into Government sewage treatment plants- Foul sewers leading into Government sewage treatment plants with microbial

treatment- Group A, B, C, D inland waters- Coastal waters, inshore waters and marine wastes of Water Control Zones

6.2 Processes of Sewage Treatment Worksa) Preliminary Wastewater Treatment- It is defined as the removal of wastewater constituents that may cause

maintenance or operational problems with the treatment operation, processes andancillary systems.

b) Primary Wastewater Treatment- In primary treatment, a portion of the suspended solids and organic matter is

removed from wastewater.

c) Secondary Treatment- It is directed principally towards the removal of biodegradable organic and

suspended solids. Disinfection is frequently included.

d) Tertiary or Advanced Treatment- This stage usually by filtering the secondary effluent to remove suspended solids

or other constituents of concern including toxic material.

6.3 Plants of Sewage Treatmenta) Cesspools- It is a watertight chamber below ground, its purpose is for temporary sewage

storage without the intention of decomposing the settled materials. It should besited at least 15 to 25m from dwelling and 15 to 25m from well/stream fordrinking water.

b) Septic Tank- Purpose of septic tank installation is to treat sewage of a domestic character from

small group of houses or larger buildings where it is not possible to connect to apublic sewage. It consists of a septic tank for primary treatment, the effluent fromwhich is given secondary treatment in a biological filter.

c) Sewage Treatment Plant- An advanced treatment process included preliminary, primary, secondary and

tertiary treatment

5 Drainage Systems

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