View source for Chapter Three: On-Site Sanitation Systems

=Chapter Three: On-site Sanitation Systems=

== 3.1 INTRODUCTION ==
Sanitation refers to the prevention of people from coming into contact with
wastes generated in homes, workplaces and public buildings, by providing
facilities and services for the safe management of human excreta to containment
and storage and treatment on-site or conveyance, treatment and eventual safe
end use or disposal (https://www.who.int/topics/sanitation/en/). On the basis
of the transport and treatment mode, there are two main types of sanitation
systems, namely; on-site sanitation (OSS) and off-site sanitation systems. Under
the off-site system, there are two sub-categories of centralized and Decentralized
Wastewater Treatment (DEWAT). This manual provides guidance for the design
of both types of systems. This chapter focuses on OSS.

== 3.2 FAECAL SLUDGE MANAGEMENT (FSM) ==
In Tanzania, it is estimated that about 90% of the population is served by OSS and
technologies which include pit latrines, septic tanks and soak-away pits (Brande
et al, 2015). These systems end up generating huge volumes of faecal sludge (FS)
which accumulates in on-site sanitation management technologies.

‘''Faecal Sludge (FS) is defined as the raw or partially digested'', ''a slurry or semisolid
material resulting from the collection'', ''storage or treatment of a combinations of
excreta and black water'', ''with or without grey water''. ''Examples of on-site sanitation
technologies include pit latrines'', ''un-sewered public ablution blocks'', ''septic tanks'',
''aqua privies'', ''and dry toilets''. ''FSM includes the storage'', ''collection'', ''transport'', ''treatment and safe end-use or disposal of FS''. ''FS is highly variable in consistency'', ''quantity'', ''and concentration''.

Recently, FS has presented itself to be one of the major sanitation management
challenges in urban areas of Tanzania. This calls for adequate and appropriate
FSM.

To better address FSM challenges, it is important to consider FSM as a sanitation
service delivery chain. In a developing country settings FSM refers to organized
programmes that provide safe and hygienic septic tank and pit latrine emptying
services, along with the proper treatment of liquids and re-use of related bio-solids
where possible. Adequate and proper FSM follows a ‘service chain’ approach. The
service chain includes the collection, storage, transport, treatment and safe enduse
or disposal of FS. FS quantity which is an important element in design of FS
treatment facility is determined as depicted on Figure 3.1.

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The complete sanitation service chain is shown in Figure 3.2. The FSM component
is specifically the emptying, collection, transport, treatment and end-use or
disposal of FS. This design manual provides a framework for the design of
sanitation systems in all its components of sanitation service delivery chain.

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=== 3.2.1 Quantification of FS ===
Deriving accurate estimates for the volume of FS produced in an area is essential
for the proper sizing of infrastructure required for the relevant collection and
transport networks, discharge sites, treatment plants, and end-use or disposal
options. The first step in designing FS (FS) treatment technologies that will meet
defined treatment objectives. This will quantify and characterise the FS to be
treated. Ideally, this should be carried out as part of the feasibility study.

Accurate estimate of FS volume is crucial for the appropriate sizing of collection
and transport systems, treatment facilities, discharging sites and disposal
options. While methods for quantification of FS are still being evaluated globally,
the following key points have been applied in the quantification of FS:<br>
(a) The amount of excreta going into the toilet depending on how many people
eat and excrete in a location,<br>
(b) How much FS is produced in addition to toilet paper and flush water to the
previous number,<br>
(c) The amount of FS accumulated by balancing accumulation and degradation
(currently it is done empirically),<br>
(d) The amount emptied from the containment,<br>
(e) The amount that is illegally discharged,<br>
(f) The actual amount delivered to the treatment plant<br>
Additionally, Figure 3.1 should be used together with the shit flow diagram
presented in Figure 3.3 to estimate the quantity of FS.

Due to the variability of FS volumes generated, it is important to make estimates
based on the requirements specifically for each location and not to estimate
values based on literature. For example, from the FS quantification diagram,
bullet six will be obtained from the number of trucks and their capacity. Then
from the shit flow diagram, this amount is equal to 11%. So, the total quantity of
FS generated is (quantity at point 6/0.11).

However, no proven methods exist for quantifying the production of FS in urban
areas, and the data collection required in order to accurately quantify FS volumes
would be too labour intensive, especially in areas where there is no existing
information.

=== 3.2.2 Characterization of FS ===
The parameters that should be considered for the characterisation of FS include
Solids - Total Solids (TS), Total Volatile Solids (TVS) and Total Suspended Solids
(TSS) concentration, Chemical Oxygen Demand (COD), Biochemical Oxygen
Demand (BOD), Nutrients, Pathogens and metals. These parameters are the
same as those considered for domestic wastewater analysis. However, it needs
to be emphasised that the characteristics of domestic wastewater and FS are very
different. In the absence of the specific standards for FS analysis, the Standard

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methods for the analysis of water and wastewater as provided for by APHA, 2012
should be used.

Table 3.1 presents examples from literature illustrating the high variability of
FS characteristics and provide a comparison with sludge from a wastewater
treatment plant.

The organic matter, total solids, ammonium, and helminth eggs concentrations
in FS are typically higher by a factor of ten or a hundred compared to wastewater
sludge (Montangero and Strauss, 2002).There is currently a lack of detailed
information on the characteristics of FS. However, research continues to
be actively conducted in this field. Research results, together with empirical
observations, will continue to increase the knowledge of FS characteristics, and
allow more accurate predictions of FS characteristics using less labour intensive
methods.

=== 3.2.3 Design Considerations for Capture and Containment ===
==== 3.2.3.1 Pit latrines, Septic Tank-Soak-away Pit Systems ====
Containment for On-site systems can be Pit Latrines of different kinds (simple or
traditional pit latrine, improved pit latrines, pour flush or eco-san. A pit latrines
can be a stand-alone or combined with a septic tank, soak-away pit. For hygiene

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reasons Tanzania encourages the use of improved pit latrines while recognising
that many people in rural and peri-urban areas use traditional pit latrines. The
most common type of on-site sanitation system in urban and peri-urban areas is
flush toilet-septic tank-soak-away pit combination. The Ministry responsible for
Health has prepared guidelines for the planning and implementation of improved
pit latrines, septic tank and pit systems for individual households, community
level, institutions and for disaster situations. This document titled “MWONGOZO
WA UJENZI WA VYOO BORA NA USAFI WA MAZINGIRA” should be referred to for
guidance whenever on-site sanitation composed of any of the aforementioned
systems is concerned. The Ministry of Health Guidelines for Improved Latrines
and Environmental Sanitation should be widely disseminated and used by LGAs
in ensuring that the Guidelines are used. The link to the document can be found
at http://www.moh.go.tz/en/enviromental-health.

==== 3.2.3.2 On-site Systems for Areas of High Water Table and Soils of Low Permeability (e.g. Clay Soils) ====
Pit latrines and septic tank, soak-away pit systems described in 3.2.3.1 can be
implemented in areas of low water table and soils of moderate to high permeability
in order to allow for the necessary treatment to occur. In areas of high water
table, latrines and soak-away pits tend to overflow during rainy seasons and may
require frequent emptying which can be very expensive. There is also concern
over public health regarding sewage running freely in the environment. It is
recommended that for such areas septic tanks or latrines should be combined
with technologies such as constructed wetlands (CW) or subsurface drains. For
areas of low permeability it is recommended to use lateral subsurface drain
connected to the soak-away pit.

Combined Septic Tank and Constructed Wetland system
Figure 3.4 and Figure 3.5 show household and institutional systems of septic
tank-constructed wetland (CW) combination applied to an area of clay soil and
high water table, respectively. The design of CW is explained in section 4.2.4.4.1
of this manual.

When infiltration from a soak-away pit is a problem due to low permeability of
the soils, increasing the area of infiltration by introducing lateral subsurface
drains also commonly known as French drains to the soak-away pit can be
applied (Figure 3.6). Subsurface drains involve excavating a trench and placing a
perforated pipe and coarse material such as gravel in the trench (Figure 3.7). The
lateral drains should follow the sloping of the land for easy draining.

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=== 3.2.4 Design Considerations for Emptying/Desludging ===
Wastewater collection technologies can in general, be categorized into either
manually operated or mechanical collection technologies. Emptying services in
low and middle-income countries are frequently a mix of mechanical tools
and manual workforce.

==== 3.2.4.1 Manually Operated Mechanical Emptying/Desludging ====
Recent innovations in human powered mechanical devices are assisting service
providers in servicing septic tanks and pit latrines more quickly, safely and
efficiently. There are four (4) common types of mechanical pumping equipment
that have been developed and tried; namely, the Sludge Gulper, the diaphragm
pump, the Nibbler, and the Manual Pit Emptying Technology (MAPET).A designer
of manually operated mechanical collection is advised to refer to Table 3.2, taking
into consideration the characteristics of FS presented in Table 3.1.

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==== 3.2.4.2 Fully Mechanized Emptying/Desludging ====
Fully mechanized technologies are powered by electricity, fuel or pneumatic
systems. They can be mounted on a frame or trolley for increased mobility, or
mounted on vehicles for emptying and transporting large quantities of sludge
over longer distances. A designer is guided to choose any the presented methods
in the Table 3.3.

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=== 3.2.5 Faecal Sludge Transport Technologies ===
There are two types of FS transport technologies, namely manual and mechanical
transport. The type of transport technology to be opted may be guided by a
number of aspects including:<br>
(a) the type of vehicle to be used including its road worthiness, maintenance
condition, licenses and permits, and where it is kept when it is not in service;<br>
(b) the type of sludge removal equipment, including hoses, pumps, augers, and
other tools of the trade;<br>
(c) the spill management equipment to be used including shovels, disinfectants,
sorbents, and collection bags;<br>
(d) the skills of the operator including training and certifications that might be
required to perform the work;<br>
(e) procedures that need to be followed including rules of the road and activities
at the treatment plant and<br>
(f) other aspects such as the use of transfer stations, worker’s health and safety,
and emerging technologies.<br>

==== 3.2.5.1 Manual Transport ====
Today, both standard carts used for general transport of materials, as well as
customized carts designed specifically for transport of FS, can be found in many
low-income countries, an example of which is shown in Figure 3.8.

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==== 3.2.5.2 Motorized Transport ====
Motorized transport equipment (Figure 3.9) offers the potential for larger load
capacities and increased speed, leading to reduced travel times and a greater
range compared to manual transport. The operation and maintenance of
motorized transport is generally more complex than that of manual transport.
However many variations are widely used in low-income countries. Before
selecting the type of transport system to use, it is important to verify that the
knowledge and skills to carry out repairs are locally available.

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=== 3.2.6 Transfer Stations ===
It has been observed that distances from the emptied OSS to a regulated
disposal facility of greater than 5km often result in illegal dumping of sewage
in creeks and rivers (Kone & Peter 2014). This illegal practice has obvious health
and environmental hazards resulting from contamination of water and attraction
of vermin and flies. In order for operators to get enough trips done in a day,
while keeping the service affordable, installation of FS transfer stations at a close
proximity to densely populated areas is essential. This approach creates a twostep
process for handling the waste matter (SSWM, 2014). Figure 3.10 indicates a
sanitation service chain of a transfer station Sanitation.

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==== 3.2.6.1 Fixed Transfer Stations ====
Fixed transfer stations can be divided into four main categories, the first of which
include ‘permanent storage tanks constructed as vault-like concrete structures.
These tanks are designed to provide storage capacity for FS over a short period
of time when there is no capacity for treatment. An example of such tanks are the
underground holding tank (UHT) reported by Boot (2007) in Accra, Ghana. With
capacities of approximately 23 m3, the UHTs were designed to provide access
to pan latrine collectors (primary transport) and vacuum trucks (secondary
transport). Care must be taken to ensure that FS is NOT stored over long periods
as it may cause transport operational challenges due to settling of sludge.

The fixed transfer station essentially serves the role of a secure, safe, storage
facility and can be designed according to the type of containers used. For example,
in one project in Ghana a concrete-lined pit within a fenced compound was used
to store the containers in order to avoid tampering, flooding or spillage. Once full
the IBCs are emptied with a vacuum truck. Figure 3.12 presents an example of a
fixed transfer station showing the top of an underground holding tank, a chain
to lock the lid, a vent pipe and a wall to prevent overflowing of excreta travelling
onto the road.

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==== 3.2.6.2 Mobile Transfer Stations ====
Mobile transfer stations consist of easily transportable containers providing
temporary storage capacity at any point near the structure being emptiedessentially
a tank fitted on a wheeled chassis. Examples of such transfer stations
include motorised collection vehicles, or tanker trailers pulled via a truck or tractor.
Figure 3.13indicates a schematic presentation of a typical mobile transfer station.

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=== 3.2.7 Design Procedures for Faecal Sludge Treatment ===
The engineering design approach provides a systematic way of setting appropriate
treatment objectives in the absence of a clear regulatory framework. The three
steps of the engineering design approach for FST plants that are briefly introduced
in this chapter are:
* Setting treatment objectives based on the final end-use or disposal option,
* Estimating quantities and qualities (Q&Q) of incoming FS,
* Evaluating selection of management solutions and treatment technologies.

The engineering design approach is an iterative process that needs to take into
account the dynamics of the full FSM chain and an enabling environment.

'''Step 1'''<br>
First, options for the final end-use or disposal of treated FS should be evaluated.
Based on the end-use, treatment objectives and corresponding performance goals
have to be defined such as can be used to work backwards and to inform design
variables for the treatment technology that can achieve these goals. Examples are
pathogen inactivation for a certain treatment product and an acceptable level of
pathogen reduction that can be measured to ensure the treatment objective. It is
important to keep in mind that over-designing treatment systems wastes money
and resources, while under-designing does not provide adequate protection of
human and environmental health.

'''Step 2'''
Second is developing an understanding of the FS quantity and quality (Q&Q) that
will be arriving at the treatment facility. Based on these estimates, technologies
can be selected and sized appropriately. For more information on methods to
determine Q&Q of FS at community to city-wide scales relevant for the design of
treatment and management solution the designer is referred to latest research
findings from the area or similar neighbourhood.

'''Step 3'''<br>
Based on the defined treatment objectives and the estimated incoming Q&Q
of FS, appropriate and feasible technology and management options can be
selected. To select the most appropriate options, the following factors will have
to be considered;<br>

(a) existing infrastructure and services,<br>
(b) available skills and capacities,<br>
(c) legal requirements and regulations,<br>
(d) social acceptance and norms,<br>
(e) operation and maintenance and<br>
(f) financial viability.<br>

Various trade-offs exist among all these factors. It is important to find an
acceptable balance from all stakeholders.

When selecting an appropriate technology for FST, it is very important to consider
whether technologies are established, being transferred from other sectors, or
are still at the innovation phase of development. Technologies are classified
based on
(a) the level of adaption,<br>
(b) research,<br>
(c) innovation and<br>
(d) expert knowledge that is required for successful implementation.<br>

Technologies are considered established if their design and operational and
maintenance guidelines can be readily recommended. Transferring technologies
is still in the process of being adapted for FSM from other sectors such as
wastewater treatment or solid waste management. Innovative technologies are
promising and potentially ready to be scaled up, but are currently still at the pilot
stage of development.

Table 3.5 summarizes FST technologies and their level of development. The
implementation of transferring and innovative technologies has an increased
risk due to there being no or less operating experience. Hence, technology
development needs to be managed during every operation. This could be
done through, for example, public or public-private partnerships or research
collaboration with research entities or universities.

The first step in the engineering design approach is defining the treatment
objectives based on the effluent and treated sludge required standards, resource
recovery and/or disposal options. Selecting technologies is also based on factors
such as cost, operational and maintenance requirements, and FS quantities and
qualities (Q&Q) that need to be treated.

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Figure 3.14 presents an example of a treatment plant process flow sheet, and the
treatment objectives achieved with each technology.

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The treatment process includes screening to remove solid waste, Anaerobic
Stabilization Reactor (ASR) for stabilization, unplanted sludge drying beds for
dewatering. The dewatered solids from the unplanted drying beds are either
co-composted or thermally dried in a solar heated greenhouse to inactivate
pathogens prior end-use. The liquid stream from dewatering unit is treated by
Anaerobic Baffled Reactor (ABR), Anaerobic Filter (AF), Planted Gravel Filter (PGF),
Pressure Sand and Activated Carbon Filter (PS &ACF) and finally disinfected in
polishing ponds (PP). Figure 3.15 indicates FS Treatment Plant (FSTP) flow sheet
as proposed by CDD society.

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==== 3.2.7.1 Settling-thickening Tanks ====
The main treatment objective of settling-thickening tanks is solid-liquid separation.
Pathogen inactivation does not occur, and both liquid effluent and settled sludge
require further treatment. Thickened sludge is normally removed after 5 to 30
days by a combination of pumps, front-loaders, and/or manually with shovels.
The loading period should be adjusted with the TSS in the supernatant layer. If
the outlet’s TSS concentration is too high, adjustments are required. There are
normally two parallel tanks to allow for sludge removal and maintenance.
Settling-thickening tanks have an inlet with a baffle to ensure quiescent flow, and
an outlet with a baffle to allow only the supernatant to pass through the outlet.
Figure 3.16 presents a schematic sketch of a typical settling-thickening tank. The
floor is commonly either sloped so sludge can be pumped out, or else flat with
access for removal by a front loader. Total suspended solids settle out into the
thickened layer, and fats, oils and grease float in the scum layer.

[[File:V2Figure3.16.JPG|669px|link=Chapter_Three:_On-Site_Sanitation_Systems]]

Prior to designing a settling-thickening tank, tests should be conducted with
Imhoff cones to determine the settleability of the specific sludge. Settleability
(recommended to be <100 mL/g TSS for settling) is determined by the Sludge
Volume Index (SVI) and is estimated by settling of 1 litre of FS in Imhoff cones or
columns for 30 to 60 minutes. The SVI is then calculated from the volume that
settles at the bottom of these Imhoff cones divided by the initial total suspended
solids concentration, CTSS.

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In general, it has been observed that sludge that is more stable, settles better
than sludge that has been stored in the containment for shorter periods of time.
Treatment performance can be measured by TSS concentrations in the effluent/
supernatant.

'''Sizing settling-thickening tanks'''<br>
The approach taken for the sizing of settling-thickening tanks is summarized in
Table 3.6.

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'''Step 1: Establish design criteria'''<br>
The design of settling-thickening tanks is based on the TSS concentration of the
incoming sludge and its settleability. FS can either be discharged directly to the
settling thickening tanks, in which case following the Q&Q method to determine
quantities and qualities and quantities would be most appropriate, or can be
loaded using other technologies. The following parameters are important:

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'''Step 2: Calculate surface area'''<br>
The surface area is calculated based on the peak flow to ensure enough time for
the particles to settle.

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Based on existing experience, the settling velocity can be estimated as 0.5 m/h in
rectangular settling-thickening tanks to treat FS with a SVI of less than 100 mL/g
TSS. The tank is designed based on the desired removal of particles from the
supernatant to the solids layer. In order for the particles to settle in the tank, a
settling velocity greater than the inflow velocity is required, as shown in Figure
3.17. The recommended ratio between width and length is between 1:10 and 1:5.
Lower settling velocities require longer tanks for particles to settle and remain in
the tank. Depending on the planned operation of the tank, the design will vary.
Manual emptying, either by truck or pump, of solids will result in different layouts
and loading and discharge cycles. The calculated area should be doubled so that
two parallel-operated settling-thickening tanks can receive discharged FS.

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'''Step 3: Sizing of thickened sludge layer'''<br>
The thickened sludge layer depth will vary with TSS concentration and will govern
the design of the settling-thickening tank.

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Vt: Volume of thickened sludge storage zone (m3)<br>
Qs: Daily inflow of FS to the settling-thickening tank (m3/d)<br>
CTSS,in: TSS concentration of incoming FS (kg TSS/m3)<br>
CTSS,out: TSS concentration of thickened FS (kg TSS/m3)<br>
e: Settling efficiency<br>
N: FS loading time (d)<br>
dt: Thickened FS depth (m)<br>

To find out how much sludge has accumulated in the thickened sludge layer,
the average daily inflow is used. However, in reality the FS loading period (N) is
also determined by a trade off or consideration of the treatment technologies
that then follow, although it should not exceed four weeks. As a principle, the
more time the thickened FS has to compact, the more challenging the removal
with pumps will be. Expected TSS concentrations in the solids and supernatant
layers can be estimated by using Imhoff cones or column testing prior to
designing the system.

'''Step 4: Determine tank depth'''<br>
The total depth is established by assumptions of the scum, supernatant and
separation layer depth. Nevertheless, excavation costs for building tanks are
commonly high and are a limiting factor in the maximum depth. Preliminary
guidelines based on studies in Accra are presented in Table 3.7.