Master of Science (Engineering) Environmental Engineering and Project Management

(1)

THE UNIVERSITY OF LEEDS

School of Civil Engineering

DISSERTATION

Submitted for the degree of

Master of Science (Engineering)

in

Environmental Engineering and Project Management

SIMPLIFIED SEWERAGE, NATURAL WASTEWATER TREATEMNET

AND WASTEWATER REUSE IN THE GAZA STRIP

By

MOHAMMED YASSIN

(2)

ii

Acknowledgment

I would like to dedicate this work to the people who I owe them every success in my life; my father, my mother, Iman, Abdul, Arwa and Ansam. Without their continuous support, encouragement, love and passion, I would never be able to reach this stage.

I would like to express my gratitude to number of individuals who supported me during my MSc course at the University of Leeds.

Firstly, I would like to thank my supervisor, Professor Duncan Mara, for his valuable guidance and continuous encouragement and support that he showed during the preparation and progress of this dissertation. His guidance, feedbacks and meetings substantially helped me to accomplish this dissertation; and in fact improved the way I think.

I am very grateful to Mrs Barbara Evans for her continuous support, encouragement and her willing to help me. I am also very thankful to number of my classmates who were always beside me when I felt homesick. Special thanks go to Thomas, Elina, Bahar and Carolina who provided a nice and friendly environment throughout the MSc course.

I would like to express a lifelong gratitude to my sponsor, Mr Akram Shakhashir, who funded me to pursue my study. I am also very grateful to Mrs Rana Diab for granting me the HQSF scholarship. Without their assistance, I would never be able to pursue my study. Special thanks also go to the British embassy in Jerusalem who facilitated my departure from the Gaza Strip.

(3)

iii

Abstract

This dissertation reveals the current situation of sanitation and wastewater treatment in the Gaza Strip. The sanitation problem in the Gaza Strip was found to be concentrated in the refugee camps which are characterized as low- income high population density areas. The refugee camps generally have no sewerage systems and the wastewater is disposed through open channels. The Gaza Strip has three main wastewater treatment plants which are old, continuously overloaded and suffers from mechanical failures due to electricity and spare parts shortage. Improper sanitation and wastewater treatment in the Gaza Strip constitute severe hazards to the public health and the surrounding environment. These also significantly contribute to increase the acuteness of water shortage in the country through contaminating the groundwater which is the only main source of water in the country.

This dissertation investigates the applicability of simplified sewerage to be applied in the refugee camps. Simplified sewerage would be an institutionally feasible, a socio-culturally acceptable, a technically appropriate and a financially attractable sanitation option to solve the intractable sanitation problem in the camps.

The dissertation also investigates the applicability of natural wastewater treatment systems, low-cost systems, in the Gaza Strip to improve the wastewater treatment sector. Six natural wastewater treatment systems were designed and compared from which WSP was found to be the most suitable one in term of construction requirements and costs, and operation and maintenance requirements and costs. The dissertation also investigates the wastewater reuse in agriculture in the Gaza Strip and its contribution to solve the water shortage problem. Wastewater reuse in agriculture would provide the agriculture sector with half of its annual demand and thus conserving large amounts of fresh water for domestic and industrial purposes. It would also result in economic benefits. The dissertation also provides a practical guide for using quantitative microbial risk analysis in wastewater reuse schemes to ensure that the risk of using treated wastewater in such scheme is within an acceptable level.

(4)

iv

List of Figures

Figure 1. 1 Geographical location of the Gaza strip ... 1

Figure 1. 2 Geographical location of Gaza’s governorates... 3

Figure 2. 1 Layout of condominial sewerage in unplanned housing blocks and in planned housing blocks ... 9

Figure 2. 2 The variation of total annual costs per household of conventional and simplified sewerage, and on-site sanitation with the population density in Natal in 1983. ... 10

Figure 2. 3 Typical layout of simplified sewerage ... 11

Figure 2. 4 Minimum depths of simplified sewers ... 13

Figure 2. 5 A schematic comparison between conventional manholes and simplified manholes. ... 14

Figure 2. 6 Parameters for open channel flow in a circular sewer.. ... 15

Figure 2. 7 Different paramters for tractive tension in a circular sewer ... 19

Figure 2. 8 Minmum depths of sewers... 23

Figure 2. 9 Combinations of ground slope and sewer gradient and solution for each case. .... 24

Figure 2. 10 Junction champers made of larger concrete diameter pipes ... 25

Figure 2. 11 All -plastic junction chamber used in Brazil. ... 25

Figure 2. 12 A vehicle mounted water-jet unit used for O&M of simplified sewers ... 26

Figure 3. 1: Typical arrangement of WSP. ... 29

Figure 3. 2 A WSP system in northeast Brazil ... 30

Figure 3. 3 Mutualistic relationship between bacteria and algae in facultative and maturation ponds. ... 38

Figure 3. 4 Diurnal variations of in-pond DO in a facultative ponds ... 38

Figure 3. 5 Conceptual mechnaism for faecal bacteria die-off ... 44

Figure 3. 6 Protection of embankments by concrete cast in situ ... 49

Figure 3. 7 Calculation of bottom and top pond dimensions from mid - depth dimensions. .. 50

Figure 3. 8 Inlet to facultative pond provided with scum box ... 51

Figure 3. 9 Pond outlet provided with scum guar ... 51

Figure 3. 10 Schematic diagram of an UASB... 54

Figure 3. 11 Influent distribution channel and distribution boxes. ... 57

Figure 3. 12 Distribution box configuration ... 57

Figure 3. 13 Examples of phase separators design ... 59

Figure 3. 14 High- rate anaerobic pond with a mixing pit. ... 62

Figure 4. 1 Aerial photo of an urban centre in the Gaza City ... 72

Figure 4. 2 An aerial photo of El Shati' camp ... 73

Figure 4. 3 Housing units layout of part of El-Shati' camp in the Gaza Strip ... 79

Figure 4. 4 Simplified sewerage network to serve part of El-Shati' camp in the Gaza Strip. .. 80

Figure 5. 1 Possible natural wastewater treatment systems ... 87

Figure 5. 2 Schematic diagram of the wastewater treatment plants to treat the wastewater of the Gaza and Middle Governorate. ... 88

Figure 5. 3 Schematic diagram of perimeter trench attachment, biogas transfer pipe and exhaust fan for collecting biogas from anaerobic pond ... 107

(7)

vii

List of Tables

Table 1. 1: Area and population of Gaza Strip's governorates ... 2

Table 2. 1: Different construction costs of selected simplified sewerage projects in Brazil in 1988... 9

Table 3. 1: Removal of pathogens achieved by WSP and conventional treatment process .... 32

Table 3. 2: Design values of λv and BOD removal in anaerobic pond at various temperatures ... 35

Table 3. 3: Minmum freeboards at different areas. ... 50

Table 3. 4: Pollutants removal mechanism in wetlands ... 63

Table 3. 5: Values of 50and for number of excreted bacterial and viral pathogens. ... 69

Table 4. 1: The population density of each camp in the Gaza Strip ... 74

Table 4. 2: Average size of household in the different camps ... 75

Table 4. 3: Design summary for the simplified sewerage network in a part of El-Shati’ camp. ... 82

Table 5. 1: Median Norovirus infection risks (pppy) from ingestion of wastewater - saturated soil determined by 10000 - trial Monte Carlo simulations ... 91

Table 5. 2: Median Ascaris infection risks (pppy) from ingestion of wastewater-saturated soil determined by 10000-trial Monte Carlo simulations ... 91

Table 5. 3: Median Norovirus infection risks (pppy) from the consumption of wastewater-irrigated lettuce determined by 10000 - trial Monte Carlo simulations ... 92

Table 5. 4: Median Ascaris infection risks (pppy) from the consumption of wastewater-irrigated lettuce determined by 10000 - trial Monte Carlo simulations ... 92

Table 5. 5: Post-treatment health protection measures and the pathogen unit reduction they achieve ... 93

Table 5. 6: Lands area requiremnts for the six proposed NWWT systems to treat 17000 m3/day of Gaza’s wastewater ... 103

(8)

viii

Principal Notations

S

YMBOLS

a area of flow, m

A area; area of flow at d/D =1 B breadth, m; Boron

C depth of cover of sewer, m; ammonia concentration, mg N/l. d depth of flow, m; dose

D diameter, mm or m; depth, m

net evaporation, mm/day

F fraction of soluble BOD; freeboard in m

gravity , m/s

ℎ depth of sewer invert, m

sewer gradient, m/m J junction

area of flow proportionally constant

hydraulic radius proportionally constant

peak flow factor; first –order rate constant for BOD removal, d

return factor

first-order rate for E. coli. Removal

sewer length, m

BOD concentration, mg/l

N manning roughness coefficient; number of equally sized maturation ponds

N number of E. coli per 100 ml

median infection dose

wetted perimeter, m

P number of population served; probability

,  flow, m/s or l/s

R hydraulic radius, m; pathogen infectivity constant.

R hydraulic radius at d/D =1; percentage of egg, BOD, or COD removal (%)

accumulation sludge rate, m / capita.year

T temperature,℃

" velocity of flow, m/s

# volume, m

$ average water consumption, l/c.d

W weight, N

% angle of flow , radians; retention time , day or hr.

∅ sewer gradient angle

' wastewater density, kg/m

)

tractive tension, N/m

* loading rate, g/m. day or Kg BOD /ha. day

3 porosity

pathogen infectivity constant.

CO carbon dioxide

HS _{bisulphide ions}

HS hydrogen sulphide

S _{sulphide ion} S7₈ sulphate

(9)

ix OH _{hydroxyl ion}

S

UBSCRIPTS a anaerobic d disease effluent f final, facultative fil. filtered

i initial, influent , individual

na non – algal m mean, maturation min minimum s surface T temperature up upward unf. unfiltered v volumetric

(10)

x

List of Acronyms and Abbreviations

BOD biochemical oxygen demand.

CAESB Water and Sewerage Company of the Federal District

CAERN Water and Sewerage Company of the State of the Rio Grande do Norte. CLIS Council of leading Islamic Scholars

CMWU Coastal Municipalities Water Utility in the Gaza Strip COD chemical oxygen demand.

CW constructed wetland

DALY disability adjusted life years DO dissolved oxygen

E.coli Escherichia coli

EG number of helminth eggs in raw wastewater FAO Food and Agriculture Organisation

FC faecal coliform FWS free water surface

GWWTP Gaza wastewater treatment plant HLR hydraulic retention rate

HRAP high rate anaerobic pond MC Monte Carlo simulation NWWT natural wastewater treatment O & M operation and maintenance OCC opportunity capital cost ppm Parts per million

PCBS Palestinian Central Bureau of Statistics. pppy per person per year

PWA Palestinian Water Authority PVC polyvinyl chloride

QMRA quantitative microbial risk analysis

SABESP Water and Sewerage utility of São Paulo in Brazil SS suspended solids

SSF subsurface-flow TSS total suspended solids WHO World Health Organisation WSP waste stabilization pond(s)

UASB upflow anaerobic sludge blanket reactor UNDP United Nation Development programme UNEP United Nation Environment Programme UNRWA United Nations Relief and Works Agency

(11)

Chapter 1 Introduction

1.1 Background of Gaza Strip

Located in the Palestinian authority territories, the Gaza strip is an elongated piece of land which is bordered by Egypt to the southwest, M

north and Negev desert to the east. It has a total length of approximately 45 km and a width varying from 6 km at the narrowest point in the north to 12 km at the southern end, giving the Gaza Strip a total area of 365 sq

location of the Gaza strip.

Figure 1.

The Gaza strip is located in an arid to semi

eastern Mediterranean climate with two distinct seasons; hot dry summer that occurs from April to August and mild winter that occurs from September to March. The

temperature reaches a minimum of Gaza Strip ranges from 200

-1

Chapter 1 Introduction

1.1 Background of Gaza Strip

Located in the Palestinian authority territories, the Gaza strip is an elongated piece of land which is bordered by Egypt to the southwest, Mediterranean Sea to the west, green line to the north and Negev desert to the east. It has a total length of approximately 45 km and a width varying from 6 km at the narrowest point in the north to 12 km at the southern end, giving the

rea of 365 sq-km (Bulter, 2009). Figure1.1 below shows the geographi

Figure 1. 1 Geographical location of the Gaza strip

The Gaza strip is located in an arid to semi-arid region and its climate is typical to that of eastern Mediterranean climate with two distinct seasons; hot dry summer that occurs from April to August and mild winter that occurs from September to March. The

temperature reaches a minimum of 4° C and maximum of 35° C. The annual rainfall in the - 400 mm, decreasing from the north to the south (EMWIS and

1

Introduction

Located in the Palestinian authority territories, the Gaza strip is an elongated piece of land editerranean Sea to the west, green line to the north and Negev desert to the east. It has a total length of approximately 45 km and a width varying from 6 km at the narrowest point in the north to 12 km at the southern end, giving the km (Bulter, 2009). Figure1.1 below shows the geographical

arid region and its climate is typical to that of eastern Mediterranean climate with two distinct seasons; hot dry summer that occurs from April to August and mild winter that occurs from September to March. The average monthly C. The annual rainfall in the 400 mm, decreasing from the north to the south (EMWIS and

(12)

Chapter (1) Introduction

2

SEMIDE, 2005). The topography of Gaza Strip is flat and rising to a maximum elevation of 105 m above the mean sea level. Most of the area is covered by Quaternary soil, with increase in clayey materials towards the east. The coastal plain, 1-2 km wide along the Mediterranean Sea, is covered with sand dunes about 20-40 m above mean sea level (Al- Agha et al. 2004).

In 2007, the year of the last census undertaken in Palestine, the population in the Gaza strip numbered up to 1,416,543 people (PCBS, 2009). The very small area with the rapidly increasing growth of population make the Gaza Strip one of the most densely populated areas in the world with a population density of 3881 person / sq-km. The Gaza strip is geographically divided into five governorates, these are; North Gaza governorate, Gaza governorate, Middle Zone governorate, Khan Younis governorate and Rafah governorate. The governorates have urban centres, rural areas and refugee camps. Refugee camps accommodate most of refugee families that were expelled to Gaza Strip as result of the occupation of Palestine. There are eight refugee camps in the Gaza strip, which occupy a total surface area of 6.26 sq-km (see Appendix1.1). Over half of Gaza’s population live in the urban centres, while 33 % of Gaza’s population or 495006 people live in the 8 camps resulting in a high average population density of 74,706 persons / sq-km in the 8 camps (Bulter, 2009). Deir al-Balah camp has the highest population density of 125000 person / sq-km while Al-Bureij camp has the lowest population density of 58712 person / sq-sq-km (UNRWA, 2010a). Table 1.1 below shows the area and population of each governorate and Figure 1.2 shows the geographical location of the 5 governorates within the Gaza Strip.

Table 1. 1: Area and population of Gaza Strip's governorates

Governorate Area (k<=) Total population at year 2007

North Gaza 61 270246 Gaza 74 496411 Middle Zone 58 205535 Khan Younis 108 270979 Rafah 64 173372 Total 365 1,416,543 Source: UNRWA, 2010b

(13)

3

Figure 1. 2 Geographical locations of Gaza’s governorates

Much of the Gaza strip is populated and approximately third of the strip is irrigated or arable. (Bulter, 2009).The Gaza strip lies above two water strata; the upper is of fresh water and the lower is of saline water (Sbeih, 1996). The groundwater in the Gaza strip is considered as the only main source of water in the area, this is directly replenished by rainfall and underground water flow from the east (Al-Ghuraiz and Enshassi, 2004). The average water consumption per capita per day in Gaza strip is 80 l/c/d (EMWIS and SEMIDE, 2005). The water situation in the Gaza strip is critical, this is because the long-term over exploitation and the continuous contamination of the only main source of water in the country together with the rapidly increasing growth of the population. The Gaza strip also has many future plans to improve the agricultural, industrial and livestock sectors, the consequence of which more water supplies will be required in the future (Gutierrez et al. 2005).

1.2 Background of sanitation and wastewater treatment in the Gaza strip

In the Gaza strip, about 35 to 40 % of the households are connected to wastewater services, these households are mainly located in cities which are served with sewers where the raw sewage is channelled to plants for treatment (PHG, 2007). Other households still use cesspools where the raw sewage is discharged into and seeps down to pollute the

(14)

4

groundwater which is the main source of the water in the country. Most commonly in camps, the raw wastewater is discharged through open channels and thus posing environmental and health hazards that adversely affect the life of the people, children in particular, living there. Currently, a significant amount of raw sewage or partially treated sewage produced in Gaza Strip is discharged into the environment either through infiltration from lagoons or cesspools to the ground and the groundwater or through direct discharge of the sewage into the Mediterranean Sea via 18 discharge points. This is mainly due to the poor performance of the wastewater treatment plants that results from insufficient electricity to effectively operate them as well as the continuous overloading of these plants (Lubbad and Alfarra, 2005). A study was undertaken by EWASH (2009) to assess the wastewater situation and the performance of the wastewater treatment plants in the five governorates of Gaza Strip. The findings of this study were as follow;

Rafah governorate

Only 65 % of Rafah governorate is connected to a sewerage network. There are 4 pumping stations which pump the wastewater to a treatment facility that consists of one treatment lagoon from which the effluent is discharged into the sea. The treatment facility receives daily flow of 8500 m /day which exceed its design capacity of 2000 m /day. This results in inadequate treatment of the wastewater with effluent characteristics of TSS 250 ppm, BOD 300 ppm and COD 550 ppm.

Khan Younis governorate

Only 25 % of khan Younis governorate is connected to a sewerage network. There are two pumping stations which pump the wastewater to storm water pond and a partly completed wastewater treatment plant that consists of settling, anaerobic and aerobic ponds. Currently, the treatment plant receives 5000 m/day and there is no discharge to the sea.

Middle Zone governorate

There is no treatment plant for this governorate and the wastewater is collected and discharged through a concrete pipe into wade Gaza which is then flows to the sea, the flow rate of untreated sewage of this governorate is 10000 m/day.

(15)

5

Gaza governorate

The Gaza wastewater treatment plant is the largest wastewater treatment plant in Gaza Strip. It consists of anaerobic pond, aerobic pond, two 9 meter high bio-towers and a polishing pond with a complete sludge management system. The effluent of the plant is discharged to the sea at three locations and due to the continuous overloading of the plant, the plant discharges untreated wastewater of 10000 m/day into the sea.

North Gaza governorate

The Governorate has a wastewater treatment plant that consists of anaerobic, aerobic, polishing and infiltration ponds. It occupies a surface area of 400000 m and receives a flow greater than 17000 m/day which is beyond its capacity.

There were many projects for upgrading the wastewater treatment plants in the Gaza strip, however all these projects were frozen due to the bad political situation that the Gaza strip suffers from.

1.3 Study justification

The improper sanitation in the Gaza strip poses a serious health and environmental hazards. Raw wastewater flowing in open channels along pathways and roads continuously threats the life of the people living in these areas, particularly children, through the spread of water and excreta – related communicable diseases such as cholera, typhoid and diarrhoea. Diarrhoea disease is one of the major causes of deaths among the Gaza strip’s population (Abu Mourad, 2004). In addition, sewage flowing in open channels can infiltrate through the soil and deteriorate the domestic water supplies and thus contaminating it. It also results in suspended solids and salts accumulation and thus destroying the soil structure and texture which in turn reduce its productivity and fertility (Sbeih, 1996). In the other hand, using cesspools - as an alternative way to dispose the wastewater - results in polluting the groundwater which is the main source of water in the Gaza strip. Al-Agha et al. (2004) showed that the lack of appropriate sanitation systems is the major environmental factor responsible for contaminating the groundwater with high levels of nitrate. The Gaza strip has been in an imposed siege since four years ago and thus it suffers from acute shortage of spare parts and energy that are necessary to effectively operate sewerage systems and the treatment plants. Shortage of necessary spare parts and energy together with continuous overloading of the plants result in disposing a high proportion of raw sewage or partially treated sewage to the environment either directly to the sea or through seepage of wastewater into the ground and

(16)

6

the groundwater and consequently cause public health hazards and adverse environmental impacts. In 2006, one of the north governorate treatment lagoons collapsed and flooded due to improper design and overloading and thus resulted in the death of five people (IRIN, 2008).

Currently, the water situation in Gaza strip is critical. The groundwater, the only main source of water, is continuously over-pumped and in many parts is contaminated. In the future more water quantities will be required to meet the rapid growth of population as well as to meet the development plans of agricultural and industrial sectors. If over pumping and contamination of the groundwater continue without a suitable intervention, the situation will be disastrous. It is the aim of this study to investigate the suitability of low cost sewerage system (simplified sewerage) as an alternative option for collecting wastewater. It also aims to propose other simple natural wastewater treatment options and investigate the most suitable one that can fit Gaza Strip nature and can produce an effluent that can be reused so to alleviate the water deficit that Gaza strip suffers from.

1.4 Objectives

The main objectives of this study are defined as follow:

To reveal and highlight the current situation of wastewater collection in Gaza strip.

To reveal and highlight the current situation of wastewater treatment in Gaza strip.

To investigate the applicability of simplified sewerage as an alternative for wastewater collection in Gaza strip.

To investigate the most suitable natural wastewater treatment technology and the applicability of using it in Gaza strip.

To investigate the reuse of wastewater in Gaza strip and how this can help in solving water problem in Gaza strip.

1.5 Thesis outline

Chapter 1 presents a background about Gaza strip. This background addresses the geographical location, climate, population and per capita water consumption. Chapter 1 also presents the current situation of Gaza strip’s sanitation, wastewater treatment and the difficulties faced to operate the current wastewater treatment facilities located in Gaza strip. It also addresses the justification of this study. Objectives and thesis outline are clearly defined in this chapter.

(17)

7

Chapters 2 and 3 will review the literature related to both simplified sewerage system and natural wastewater treatment. Chapter 2 will present an introduction to simplified sewerage, its origin and development, also design standards and design equations will be highlighted. Chapter 3 will present an introduction to waste stabilization ponds, the different types of waste stabilization ponds and the design of each one of them. Physical design, operation and maintenance of waste stabilization ponds system will be illustrated in this chapter. Other natural wastewater treatment units (constructed wetlands, UASBs and high rate anaerobic ponds) will be reviewed as well as wastewater reuse.

Chapter 4 will examine the applicability of simplified sewerage in the Gaza Strip; mainly camps are targeted. The applicability will be examined based on the camps characteristics (i.e. building layouts, population density, costs, socio-cultural and other factors). A simplified sewerage network in one of the camps – a high population density area – will be designed in this chapter by using a prepared excel sheet as to provide a practical example.

Chapter 5 will examine the applicability of natural wastewater treatment in the Gaza strip in term of climate, area requirements. Different natural wastewater treatment systems will be designed to treat a proportion of Gaza strip’s wastewater to investigate the area requirements of these systems and thus their suitability for Gaza Strip. The different systems will be compared in term of area requirements, construction requirements and costs, O&M requirements and costs, and the potential to produce energy so that to come up with the most suitable system for Gaza Strip. Reuse of wastewater in the Gaza Strip will be investigated and how this can contribute to solve the water problem in the strip. Quantitative microbial risk analysis will be applied so as to provide a guide for ensuring that the risk of using treated wastewater water is within an acceptable level.

(18)

2

Literature review: Simplified Sewerage

8

Chapter 2 Literature Review: Simplified Sewerage

2.1 Introduction to simplified sewerage

Simplified sewerage is an offsite sanitation technology that is designed to receive and convey all household wastewater including settleable solids. It was first developed in north east Brazil in the early 1980s by CAERN, the Water and Sewerage Company of the state of the Rio Grande do Norte, as a technically feasible and financially affordable solution for the intractable problem of sanitation provision in low-income high-density areas (Mara, 1996a). Conceptually, simplified sewerage is a conventional sewerage stripped down to its hydraulic basis through efforts made to eliminate the unnecessarily conservative design features that have accrued with the design of conventional sewerage during the last 100 years. This results in a lower-cost sewer systems with flatter, shallower and smaller sewers with simpler and fewer manholes (i.e. inspection units instead of large expensive manholes) (Mara, 1998). Simplified sewers can be either laid inside the housing blocks and in this case it is known as condominial sewerage or laid outside the housing blocks under pavements on both sidewalks of the street rather than conventional sewerage where the sewers are laid in the middle of the street (Mara, 1998). The condominial sewerage, also known as in-block system, refers to that sewers are routed in private lands, through either front yards or back yards and it is less expensive than in-street system (Mara et al. 2001). Sinnatamby (1983) showed that condominial sewerage can be laid in planned and unplanned areas (see Figure 2.1). The flexibility of laying sewers at different routes has significant contribution in saving costs over conventional sewerage. In-block systems, back-yard sewerage in particular, can significantly reduce the required length of sewer and thus saving costs. Laying the sewers away from heavy traffic loads result in shallower excavation depths and thus costs are further reduced (Mara et al. 2001). The costs of simplified sewerage are low. This is due to the use of small diameter of pipes, the use of shallow excavation depths, the use of flatter gradients of sewers than that used with conventional sewerage systems and the use of simple inspection units instead of expensive large manholes (Mara, 1998). Thus, simplified sewers have shown to be significantly less costly than conventional sewers.

(19)

Chapter (2) Literature Review: Simplified Sewerage

9

Figure 2. 1 Layout of condominial sewerage in unplanned housing block (left) and in planned housing blocks (right). Source: Sinnatamby (1983)

In Natal in north east of Brazil, the capital cost of simplified sewerage per household in 1980 with $1500 per household for conventional sewerage. As a consequence, CAERN was able to recover its cost over a period of 30 years through surcharging the monthly water bill by 40% compared with 100% that was required for conventional sewerage. This was affordable to the households as they were required to pay only 1.5$ per month which is equivalent to the 40 % of the monthly water charge (Mara et al. 2001). Cost savings between 20 to 50 % have also been reported in many places. In the state of Sao Paulo, the first simplified sewerage project had achieved a cost saving of 30 % over conventional sewerage; while after 8 years of experience, cost savings are estimated to be close to 40 %. The SABESP, the water and sewerage utility of Sao Paulo in Brazil, estimated the per capita construction costs of simplified sewerage and conventional sewerage for a small town, these costs exclude costs of house connections and treatment and they are based on prices of 1988. For that town, the cost of simplified sewerage was US$80-150 per capita; while it was US$ 150-300 per capita for conventional sewerage (Bakalian et al. 1994). Table 2.1 below shows the different construction costs (based on 1988 prices) for number of selected simplified projects in Brazil.

Table 2. 1: Different construction costs of selected simplified sewerage projects in Brazil in 1988

City Sao Paulo Cardosa Coraodos Toledo

Total cost $ 1897000 $ 48000 $ 68000 $ 3762000

Cost / metre $ 76 $ 13 $ 8 $ 21

Cost / capita $ 151 $ 51 $ 87 $ 59

Source: Mara (1996)

The cost of simplified sewerage can be further reduced by getting the users to contribute in excavating sewer trenches provided that there is a technical assistance. This was implemented in Orangi, Pakistan and due to users contribution the cost of simplified sewerage was 40$ per household which is much lower than the cost in north east Brazil (Mara et al. 2001). Mara

(20)

10

(1998) stated that simplified sewerage systems are most appropriate for low-income, high- density areas that are served by on-plot level of water supply and where there are no spaces for on-site sanitation systems or for septic tanks of settled sewerage. Simplified sewerage, depending on the local population density, can be cheaper than on-site sanitation systems (Mara et al. 2001). Figure 2.2 below is a finding of the city of Natal in north east Brazil in 1983, which shows that simplified sewerage becomes cheaper than on-site sanitation system at population density larger than 160 persons per hectare. Given the extensively high rate of urbanization that has created low-income, high density areas in many developing countries, Mara (1998) believes that simplified sewerage will be often the only technically-appropriate, institutionally feasible and financially affordable sanitation technology to expand the sanitation coverage in the world.

Figure 2. 2 The variation of total annual costs per household of conventional and simplified sewerage , and on-site sanitation with the population density in Natal in 1983. Source: Sinnatamby (1983).

Simplified sewerage has been widely and successfully used in Brazil and elsewhere in Latin America, for example, it is implemented in Nicaragua, Paraguay, Colombia and Bolivia. In Asia and Africa, it is much less well known. In Asia, it has been successfully used in Malang, Indonesia, India and in Karachi, Pakistan. In Africa, it has been used in few scales in South Africa (Mara et al. 2001).

(21)

11

2.2 Design of simplified sewerage

2.2.1 Design criteria of simplified sewerage

Layout

The layout of simplified sewerage is based on dividing the network into smaller network systems. Thus, areas to be served are initially defined, where feasible, by individual drainage basins. Each drainage basin has its own collectors and units .When funds and resources become available, the individual networks may be connected together through a common collector for conveyance to a central treatment plant. Furthermore, the sewers are laid away from heavy traffic loads, usually under sidewalks on both sides of the street or backyards (Figure 2.3) rather than in the centreline of the street and thus construction costs are significantly reduced due to the optimization of pipe lengths and minimization of cost of excavation depths and cost associated with pavement restoration (Bakalian et al. 1994).

(22)

12 Design Period

Design flows are projected either based on design periods or expected saturation population of the area under consideration. The concept of design periods are not used when dealing with fully developed areas. Alternatively, the saturation population for a certain drainage basin can be estimated by assuming 5 people per household and by predicating the future land use patterns. In case of not using saturation population method, a design period of not more than 20 years is recommended (Mara, 1996a) rather than the longer design periods of conventional sewerage that ranges from 20 – 25 years. Bakalian et al. (1994) recommends the use of such shorter periods to avoid problems associated with uncertainties of predicting the population growth and their water consumption and the high costs of maintaining large sewers under low flows. Shorter design periods also result in lower capital costs of the sewerage projects and thus facilitating financing process or motivating authorities having certain budget towards expanding sewerage coverage to more areas.

Design flow

As water supplied is partially lost during domestic activities (i.e. Garden watering, house cleaning) or leakage, the wastewater flow quantities are lower than the water supplied quantities. Thus, it is important to keep records of water supplied each day and daily fluctuations of water consumption in order to accurately determine the projected flow of wastewater. To avoid improper estimates of design flows, it is essential to obtain information about water use in the area under consideration rather than depending on information from areas of similar characteristics and thus the returned water or return coefficient can be accurately determined. The wastewater design flow is calculated based on the quantity of returned water and a peak factor. The return coefficient varies depending on the characteristics of the area and the population (i.e. the return coefficient in arid in USA is .40; while in Sao Paulo it is estimated to be .80). The peak factor that has been successfully used in Brazil for simplified sewerage design is 1.80 rather than peak factors ranging from 2-3 that is used by industrialised countries. In addition to these estimates an allowances of 0.50 – 1 l/s per km of pipe are added for clear water infiltration. In case of lack to information about water use, a minimum flow of 1.5 l/s is used as wastewater design flow for simplified sewerage (Bakalian et al. 1994; Mara et al. 2001).This value approximately equals the flow produced from flushing a toilet (Mara et al. 2001).

(23)

13 Minimum diameter

Simplified sewerage uses smaller minimum diameter than that used with conventional sewerage. The use of smaller diameter is effective in the upper part of the system where wastewater flow is low; this can result in higher velocities, greater flow depth and effective cleansing (Bakalian et al.1994). In Brazil, laterals or branch sewers of 100 mm diameter are being used for a maximum length of 400mm. These pipes of 100-mm diameter are usually laid under unpaved streets of peri-urban areas (Bakalian et al. 1994; Mara, 1996a).

Depth of Sewers:

Minimum depth of sewer should be appropriate enough for two purposes (Bakalian et al. 1994);

a) To make house connections

b) To provide the pipe with a preventive layer of soil over its crown and thus to protect it from structural damage that result from external loads (i.e. heavy traffic).

For simplified sewerage, the following minimum depths of sewers are recommended; 0.65m under sidewalks, 0.95-1.50 m below residential areas (depending on the traffic load and the distance from the street centreline), and 2.5 m below street of heavy traffic. Figure 2.4 below depicts the minimum sewer depths for different cases. In setting sewers invert levels, building elevations are not considered. If buildings elevations are too low to join the network, the property owner is responsible for finding other alternatives to make the connection (Bakalian

et al. 1994).

(24)

14 Manholes

Manholes are considered as one of the expensive components of sewerage systems. In Brazil, expensive conventional manholes are replaced with either cleanouts, simplified manholes or buried boxes (see Appendix 2.1, 2.2 & 2.3). Simplified manholes are of small diameters 0.60 -0.90 m rather than 1.5 m used with conventional manholes. The reduction in manhole sizes is due to the availability of new cleaning equipments and to the shallower depths that together eliminated the need for personal maintenance to enter the manholes. Simplified manholes are only used at major junctions (Bakalian et al. 1994). Figure 2.5 below shows the difference between conventional manholes and simplified manholes

Figure 2. 5 A schematic comparison between conventional manholes and simplified manholes.

Source: Bakalian et al. (1994)

Buried boxes or chambers replace large manholes at changes of slopes or direction (Bakalian

et al. 1994; Mara et al. 2001) (see Appendix 2.4 and 2.5). House connection is modified to

serve as inspection units; a small box is installed under the walkway and is connected to the sewer with a curve of 45° where the jetting hose or cleaning rod can easily be introduced

through this box. The new guidelines for design of manholes significantly reduce the costs of sewerage system (Bakalian et al. 1994).

(25)

15 Sewer Slopes

The minimum slope of the sewer is calculated based on a tractive tension value of 1N/m (Mara, 1996a). This will be shown in the next section.

2.2.2 Theory and design of simplified sewerage

The flow in simplified sewers resembles open channel flow, which means that there is a free space always above the surface of the wastewater flowing in the sewer. The area of the flow and the hydraulic radius are essential parameters for the design of simplified sewers. These two parameters vary according to the depth of the flow (Mara, 1996b; Mara et al. 2001). Figure 2.6 shows the parameters of open channel flow in a circular sewer.

Figure 2. 6 Parameters for open channel flow in a circular sewer. Source: Mara (1996b).

Where (Mara, 1996b),

a: is the flow area, expressed in <=;

p: is the wetted parameter, expressed in <; b: is the flow breadth, expressed in m; @: is the angle of flow, expressed in radians; A: is the depth of flow, expressed in m; and

D: is the sewer diameter, expressed in m

The hydraulic radius (r), known also as hydraulic mean depth, is defined as the area of the flow (a) divided by the wetted perimeter (p).

(26)

16

The dimensionless ratio (d/D) is known as the proportional depth of the flow, in simplified sewerage this ratio ranges from 0.2 to 0.80 rather than conventional sewerage where this value is restricted to range from 0.50 to 0.75. Thus, more of the hydraulic capacity of the pipe is used with simplified sewers. The lower limit (d/D = 0.20) in simplified sewers allow for sufficient velocity of the flow to prevent deposition of solids, mainly, at the start of the design period, while the upper limit (d/D = 0.80) allows for sufficient ventilation, mainly at the end of the design period. The following equations represent the trigonometric relationships between the different parameter, which were derived based on Figure 2.6 (Mara, 1996b; Mara et al. 2001): - Angle of flow: @ = 2DE [1 − 2 IJ KL] (2.1) - Area of flow: N = O[(@ − Q@)/8] (2.2) - Wetted perimeter: p = @O/2 (2.3) - Hydraulic radius: r = IK₈L [1 − ISTUV_V L] (2.4)

- Breadth of the flow:

b = O Q(V

) (2.5)

When the sewer is flowing just full (d=D), then p = P = W O , a =A= ( W/4)O and r = (a/p)

= D/4.

In designing simplified sewer, N and Y can be expressed in the following equations;

a = O (2.6)

r = O (2.7)

The coefficient and are function of @ and can be given from equation 2.2 and 2.4 respectively as: = Z(@ − Q @) (2.8) = 8(1 − (( Q @)/@) (2.9)

(27)

17

Since the flow in simplified sewers resemble open flow channel, the basic equation used in the design of simplified sewerage is Gauckler-Manning equation which relates the flow velocity in a sewer with the hydraulic radius and sewer gradient as follow (Mara, 1996b; Mara et al. 2001):

" = I

UL Y/ . (2.10) Where, " = the velocity of flow at proportional depth (d/D), expressed in m/s. n = roughness coefficient, dimensionless.

Y = hydraulic radius at proportional depth (d/D), expressed in m. = gradient of sewer, dimensionless.

Since flow = velocity × area

= I_UL N Y/. (2.11)

Using expressions of N and Y given in equation 2.6 and 2.7 in equation 2.11, equation 2.11 becomes as follow;

= I_UL O (O)/ . (2.12) Wastewater flow

The daily peak flow of wastewater used for designing simplified sewers can be expressed in the following formula (Mara, 1996b; Mara et al. 2001):

= \ $/ 86400 (2.13) Where, is the daily peak flow, expressed in l/s.

is thepeak factor , which is equal to daily peak flow divided by the average daily flow.

is the return coefficient , which is equal to wastewater generated divided by water consumption.

(28)

18

$ is the average water consumption in area under consideration, expressed in litre per capita per day (l/c/d).

86400 is used to convert the flow from day to seconds.

The factors and vary according to the conditions of areas to be served, however it has been found in Brazil that values of 1.80 for and 0.85 for are suitable design values for designing simplified sewers. Thus, equation 2.13 becomes as follow (Mara, 1996b; Mara et

al. 2001):

= 1.8 × 10 \$ (2.14)

Two approaches are used to design simplified sewers; minimum self- cleansing velocity and minimum tractive tension. Minimum self -cleansing velocity is the minimum velocity of flow that ensures transport of solids, a minimum self cleansing velocity of 0.50 m/s has been successfully used in Brazil (Mara, 1996b). However, the design approach of minimum tractive tension (Machado Neto and Tstutiya, 1985) has proven to result in sewers of shallower gradients and thus reducing the construction costs due to less excavation requirements (Bakalian et al. 1994; Mara, 1996a; Mara, 1996b). Bakalian et al.(1994) also stated that other authors (Yao, 1974; Machado, 1985) have recommended the use of minimum tractive tension approach in calculating the minimum slope of sewers as it is more cost-effective than self-cleansing design approach. Thus, simplified sewers are designed based on maintaining a tractive tension of 1 N/m, this value is sufficient to re-suspend solids of 1-mm size (Bakalian et al. 1994).

Tractive tension

Tractive tension (also known as boundary shear stress) is defined by Mara et al. (2001) as “the tangential force exerted by the flow of wastewater per unit wetted boundary area”. It is expressed in N/m or Pascal and detonated by the symbol ). For a wastewater mass flowing in a circular sewer with a cross sectional area of a m, a length of l m and a wetted perimeter of p (as shown in Figure 2.7), the tractive tension is represented by the component of the weight of this wastewater mass in the direction of flow (^ Q∅, expressed in Newton) divided by its wetted boundary area (the area that the wastewater mass contacts the sewer boundary = pl ) (Mara et al. 2001):

(29)

19

Since the density of the wastewater is the wastewater mass divided by its corresponding volume and the weight is the mass multiplied by the gravity, the wastewater weight can be given by the following formula,

∴ ^ = 'N (2.16)

Where ' is the wastewater density, expressed in kg/m.

a is the cross sectional area of the flow, expressed in m

is the length of the wastewater flowing in the sewer, expressed in m g is the acceleration due to gravity, expressed in m/s

By substituting W in equation 2.15 by its expression in equation 2.16,

∴ ) =

abc def ∅

gb (2.17) Since, the hydraulic radius (r) is equal a/p

∴ ) = ' Y sin ∅ (2.18)

When ∅ is so small, sin ∅ is equal to tan ∅ which is known as the sewer gradient and is denoted by . Thus, equation 2.18 becomes as follow,

∴ ) = ' Y (2.19)

Figure 2. 7 Different paramters for tractive tension in a circular sewer. Source : Barnes et al. (1981)

(30)

20

By substituting Y in equation 2.18 by its expression in equation 2.7 and rearranging the equation for the value of D,

O = ()/' )/ (2.20)

Substituting D in equation 2.12 by its expression in equation 2.19 and simplifying the equation, thus;

= I_UL ()/' )Z/ /k (2.21)

One of the key features of the design of simplified sewerage is the design concept developed by Eugênio Macedo; the sewer gradient is designed based on the flow at the start of the design period (_T), while the sewer diameter is designed based on the flow at the end of the design period (_l). In low income areas the value of _l can be up to five times of the value of

T (Sinnatamby, 1983). Minimum sewer slope

By substituting )_mTU for ) and _mTU for in equation 2.21 and rearranging the equation, the minimum sewer gradient _mTU can be expressed in the following formula;

_mTU = [I_UL ]k/[)_mTU/' ]k/k/ (2.22)

Using d/D = .20 (the lower limit of the proportional depth used in the design of simplified sewer) in equations 2.1, the value of @ is 1.85 radians. Thus, using this value of @ in equations 2.8 and 2.9, the values of and are .1118 and .1206 respectively. Using in equation 2.21these values of and with n = 0.013, g = 9.81 n/s, ' = 1000 /n and with a design value of )_mTU = 1N/n (which is a good design value that is sufficient to ensure re-suspended of solids of size of 1mm), the equation can be rewritten as follow (Mara, 1996b; Mara et al. 2001);

_mTU = 2.33 × 108k/ (2.23)

As illustrated earlier, the sewer gradient is designed for the flow at the start of the design period (_T) and therefore q used in equation 2.23 should be _T .The value of q also is subject to be greater than the minimum daily peak flow (1.5 l/s), otherwise a value of 1.5 l/s should be used (Mara et al. 2001);. In equation 2.23, the unit of q is m/s. For values of q in l/s, _mTU is given by the following formula;

(31)

21

mTU = 5.64 × 10k/ (2.24)

Sewer Diameter

By substituting _mTU for  in equation 2.12 and rearranging the equation for D, the equation

becomes as follow (Mara et al. 2001);

O = Q/Z/Z/8(/_mTU.)/Z (2.25)

Where, D is the sewer diameter in m and q is the flow in m/s. Since the diameter of

simplified sewer is designed based on the flow at the end of the design period (l) , the value of used in equation 2.25 to determine the sewer diameter is l (Mara et al. 2001).

Design procedures

Simplified sewers can be designed by following the sequence of the following steps (Mara et

al. 2001):

• Using equation 2.13, calculate the peak flow at the start of the design period in l/s (T) and the peak flow at the end of the design period (l). A design period of no more than 20 years is suitable for designing simplified sewers. The increase of l over _T is due to either an increase in population number (p) during the design period or an increase in water consumption ($, lcd), or both. In a fully developed area,

where there is no increase in water consumption , T may equal l

For a section of sewer being designed, the (P) in equation 2.13 is the number of population in households connected above the downstream end of that section.

• Calculate the minimum sewer gradient ( mTU ) from equation 2.24 with q=T . This value of _T is subject to be greater than the minimum daily peak flow of 1.5 l/s , otherwise a value of 1.5 l/s for T is used.

• Calculate the sewer diameter (D) using equation 2.25, with = l (in m/s) which is also subject to a minimum value of 0.0015 m/s. The diameter calculated from this equation is unlikely to be a commercial diameter and thus next larger diameter which is commercially available is selected (i.e. if D so calculated is 82 mm, then select a sewer of 100 mm diameter).

(32)

22

There are many design charts that are in use for designing simplified sewerage. A design chart for designing simplified sewers is given in appendix 2.4. This chart is based on manning equation and it correlates d/D with "/. and /. at different commercial diameters (D=100, 150, 225 and 300 mm). Thus, there are five columns in this chart; the first one on the left for d/D values and four columns for the commercial diameters mentioned above. Each column of the four is subdivided into two columns for "/. and /. values. Simply, the sewer diameter can be calculated as follow (Mara, 1996b);

o Calculate the value of _T and _l from equation 2.13,

o Calculate the value of _mTU from equation 2.24 with q=_T. The value of q should be larger than minimum peak daily flow (1.5 l/s), otherwise a value of 1.5 l/s should be used

o Calculate the value of _l/_mTU. , then from the design chart find this value where d/D is close but not larger than .80 (Note: when starting with the chart, block out parts not in use ; d/D < 0.20 and d/D >.80 as the proportional depth in simplified sewers ranges from 0.20 to 0.80)

o The Diameter (D) is at the top of the column where the value of _l/_mTU. was found.

The units of the chart parameters are as follow; v in m/s, q in m/s and i in m/m. 2.3 Construction of simplified sewerage

Good construction of simplified sewerage avoids operational problems and ensures long-term sustainability of the system (Watson, 1995). Good construction of simplified sewerage is the same as practised in conventional sewerage; however laying sewers of small diameters and shallow depths should be given special care (Mara et al. 2001). Mara et al. (2001) also recommends training of local contracting companies inexperienced in construction of simplified sewerage as well as practising effective construction supervision for ensuring good construction.

Maps detailing ground elevations of the area to be served should be available during the design process, these are important for calculating the ground slopes (S) and thus sewers can be calculated to fit the topography of the area considered (Mara et al. 2001). In practice, the ground slope may be equal to, greater than, much greater than or less than the minimum sewer slope _mTU. In addition, invert level of the upstream end of the sewer considered may

(33)

23

be greater than or equal to the minimum depth permitted ℎ_mTU (Mara et al. 2001). ℎ_mTU is expressed in the following equation;

ℎ_mTU = p + O (2.26) Where, ℎ_mTU is the minimum depth permitted, expressed in m. (see Figure 2.8) C is the minimum cover, in m

D is the sewer diameter, in m

Figure 2. 8 Minmum depth of sewer. Source:Mara et al. (2001)

Therefore, in practice, six combinations of ground slope and sewer gradient are likely to be encountered (Mara et al. 2001). Figure (2.9) summarizes these combinations and proposes solution for each case regarding to the calculation of the sewer slope and invert level of the downstream end of the sewer. Both of these parameters are essential when laying sewers in sanitary projects.

The same types of pipe materials used for conventional sewerage are also used for simplified sewerage. In Brazil, the most commonly types of pipe materials used for simplified sewers are vitrified clay, polyvinyl chloride (PVC) and asbestos cement (Bakalian et al. 1994). However, Mara et al. (2001) recommends not using sewers made of asbestos-cement due to the potential generation of hydrogen sulphide (HS) in sewers as asbestos cement is so vulnerable to HS and can quickly be corroded. Bakalian et al. (1994) and Mara et al. (2001) sees that the most preferred type is (PVC) pipes as they have advantages of longer lengths that can be jointed more properly and thereby infiltration is minimised (i.e. groundwater ingress is less).

(34)

24

Figure 2. 9 Combinations of ground slope and sewer gradient with sloutions for each case. Source: Mara et al. 2001

As shown in Section 2.2.1 above, simplified sewers are laid at shallower depths and thus simpler junction boxes or inspection chambers can replace large expensive manholes. There are different options of these chambers or boxes. For instance, junction chambers made of pre-cast cylindrical concrete sections are used in Guatemala (see Figure 2.10). In Brazil, simple inspection chambers made of bricks were used. Other option is the all-plastic junction unit (as shown in Figure 2.11) that was more recently developed in Brazil (Mara et al. 2001). Junction chambers are installed at every connection to the system, while inspection chambers are installed at changes of direction or slopes (Bakalian et al. 1994; Mara et al. 2010 ) and at intervals not larger than 30 m in case of condominial sewers and 100 m in case of public collector sewers (Mara et al. 2010 ).

(35)

25

Figure 2. 10 Junction champers made of larger concrete diameter pipes, used in Guatemala.

Source: Mara et al. (2001)

Figure 2. 11 All -plastic junction chamber, used in Brazil (manufactured by Tigre S.A).

Source: Mara et al. (2001)

2.4 Operation and maintenance of simplified sewers

Since simplified sewerage came in use, it has not been reported any significant problems or problems regarding excess HS generation (Bakalian et al. 1994). Operation and maintenance of simplified sewerage are very simple and can be either carried out by the sewerage authority itself or by contracting a small local contracting company that can provide a maintenance team. In addition, maintenance of the simplified sewers system can be assigned to residents of the area served provided that they were subjected to a training programme and that there is a technical support from a technical office, where necessary (Mara et al. 2001). The maintenance requirements for simplified sewerage are similar to those practised with conventional sewerage. Minimum maintenance requirement consists of repairs, flushing, cleaning and continuous supervision of connections (Bakalian et al. 1994). For a successful

(36)

26

operation of a sewerage scheme, it is essential to have a maintenance programme which at least should include the following (Bakalian et al. 1994);

Continuous inspection of the network and recording of the information obtained in a data base. Thus, these data can be analysed to give guidance for maintenance crews regarding to how often and where maintenance actions should be carried out and the type of maintenance which would be most effective.

Prompt corrective actions to be carried out such as the removal of foreign materials accumulation.

Occasional flushing and cleaning of the sewer line.

Different cleaning equipments and methods are in use for the maintenance of simplified sewers, the choice of agency or sewerage authority is based on its budget, facilities and staff experience .The most commonly cleaning equipments used in Brazil are flushing equipments and rodding machines with the former being used more increasingly (Bakalian et al. 1994). In Brasilia and federal district, for example, the water and sewerage company of the federal district (CAESB) uses water-jet unit mounted in a vehicle (see Figure 2.10) for cleaning simplified sewers (Mara et al. 2001). Blockage in a sewer then can be removed by introducing a high velocity water jet at the junction box just upstream the blockage which is then removed down the sewer to the downstream junction box where it can be caught and removed.

Figure 2. 12 A vehicle mounted water-jet unit used for O&M of Simplified sewers in Brasilia

To ensure long term sustainability of simplified sewerage, Mara et al. (2001) believes that there should be an effective partnership between sewerage authority and the community served so that each party is clear about its duties and responsibilities. It is also essential to

(37)

27

conduct community education programmes that aim to raise the awareness of the residents in relation to what should not be disposed via simplified sewers and how to report leaks and blockages. In addition, the tariff structure of simplified sewerage should be ‘adequate’ that the sewerage authority receives sufficient income from the levied monthly charges, but also it should be affordable to the residents. Long term sustainability can also be ensured by good design, good construction and effective maintenance.

(38)

3

Literature review: Natural Wastewater Treatment and

Wastewater Reuse

28

Chapter 3 Literature review: Natural Wastewater

Treatment and Wastewater Reuse

3.1 Natural wastewater treatment

Crites et al. (2006) defines natural treatment systems as those processes which depend mainly upon their natural components or responses (i.e. gravity force for sedimentation) in achieving the desired purpose or wastewater treatment goal. The most common approach in designing these systems is to maximise the utilisation of their natural components, which in most cases results in less energy requirements and lower costs to build and operate than conventional treatment alternatives. Crites et al. (2006) also categorises natural wastewater treatment systems in three main categories; aquatic (i.e. waste stabilization ponds-WSP), wetlands (i.e. constructed wetlands) and terrestrial (i.e. dependence upon the soil matrix in the treatment process). Each category depends on its natural chemical or physical response as well as its own biological components in the treatment process.

These natural systems might include piping and pumps for wastewater conveyance but certainly would not rely on external energy as an exclusive source in maintaining the main treatment responses (Crites et al. 2006). For example, they do not depend on electrometrical parts for their operation (i.e. aerators) and hence oxygen required to oxidise the organic compounds is provided by other natural sources such as photosynthetic activity of plants in constructed wetlands and of algae in waste stabilization ponds. This reliance upon sunlight- related activities will result in greater land areas than that required for conventional treatment systems and thus it is most appropriate for developing countries where there is often sufficient land (Mara et al. 2003; Mara, 2003).

This section will present the literature review related to the following natural wastewater treatment units; waste stabilization ponds, high rate anaerobic ponds, UASBs and constructed wetlands.

3.1.1 Waste stabilization ponds

Waste stabilization ponds (WSPs) are shallow, large man-made basins bounded by earth embankments where raw wastewater is received and treated by completely natural processes

(39)

Chapter (3) Literature Review: Natural Wastewater Treatment and Wastewater Reuse

29

that involve algal and bacterial activates. Direct solar energy is the only form of energy they use and thus there is no need for expensive electromechanical equipments used with conventional treatment systems. Their hydraulic retention times are much longer than conventional treatment systems (expressed in days rather than hours) due to their slower oxidation rates (Mara, 1992; Mara, 2003; Mara and Peňa, 2004).

There are three main types of WSP; anaerobic ponds, facultative ponds and maturation ponds. Since it is widely observed that a series of ponds produce an effluent of better quality than that produced from one large pond of the same size (Mara, 2003), the different types of WSP are arranged in one or more series within a WSP system- the first in the series is usually anaerobic pond followed by facultative pond, and finally by one or more maturation ponds depending on the desired effluent quality (Mara, 1992; Mara, 2003; Mara and Peňa, 2004). Figure 3.1 shows the typical arrangement of WSP system and Figure 3.2 shows a WSP system implemented in Brazil. Anaerobic and facultative ponds are mainly designed for removal of BOD (biochemical oxygen demand), while maturation ponds are mainly designed for the removal of faecal viruses (i.e. Rotavirus, Norovirus) and faecal bacteria (i.e.

Salmonella spp.) Some pathogens removal (particularly the removal of Vibro cholera) occurs

in anaerobic and facultative ponds, which also remove most of helminth eggs; and some of BOD removal takes place in maturation ponds that also remove some nutrients, N and P. (Mara, 1992; Mara, 2003; Mara and Peňa, 2004).

Master of Science (Engineering) Environmental Engineering and Project Management

THE UNIVERSITY OF LEEDS

School of Civil Engineering

DISSERTATION

Submitted for the degree of

Master of Science (Engineering)

in

Environmental Engineering and Project Management

SIMPLIFIED SEWERAGE, NATURAL WASTEWATER TREATEMNET

AND WASTEWATER REUSE IN THE GAZA STRIP

By

MOHAMMED YASSIN

Acknowledgment

Abstract

Table of Contents

List of Figures

List of Tables

Principal Notations

S

)

S

List of Acronyms and Abbreviations

Chapter 1 Introduction

Chapter 1 Introduction

1

Introduction

2

Literature review: Simplified Sewerage

Chapter 2 Literature Review: Simplified Sewerage

∴ ) =

3

Literature review: Natural Wastewater Treatment and

Wastewater Reuse

Chapter 3 Literature review: Natural Wastewater

Treatment and Wastewater Reuse