NAME: EDWIN KARANJA NG’ANG’A
REG NO: J173/3040/2018
SCHOOL: ENGINEERING
DEPARTMENT: CIVIL ENGINEERING
UNIT CODE: ECV 308
UNIT NAME: SOIL MECHANICS II
LECTURER: DR. MARCLUS MWAI
ASSIGNMENT: SANITARY LANDFILL DESIGN
Introduction
A sanitary landfill is an engineering method of disposing solid waste on land. As
such, landfills are required to protect the environment by spreading waste into thin
layers and compacting them into the smallest practical volume. By day’s end, all
waste is then covered with earth.
In my design of a sanitary landfill my focus will be Kiambu county, with it being my
home county. Kiambu county is adjacent to the northern border of Nairobi county and
has a population of 2,417,735 (according to the 2019 population census). The county
is 40% rural and 60% urban owing to Nairobi’s consistent growth Northwards. SWM
service is provided by both the county and private sector.
The landfill project is essential to assure proper solid waste management within the
county and to mitigate exposure of the environment and humans to the detrimental
effects of solid wastes currently disposed in open dumpsites by containing and
isolating the waste.
1. Sizing
The following data was used:
Waste Generation
:
Design Life
:
Water-table
:
Average Total Precipitation:
370 tons per day (current)
15 years
10m below ground surface
1200 mm per year
Landfill capacity, Landfill height, Landfill area
(a) Current waste generation per day = 370 t/day
(b) Estimated waste generation after 15 years = 600 t/day
(c) Total Waste Generation in 15 Years
= 0.5 (370 + 600) x 365 x 15
= 2.65 x 10^6 tons
(d) Total Waste Volume (assumed density 0.85 t/cu.m.)
Vw= (2.65 x 10^6 )/0.85
= 3.118 x 10^6 cu.m.
(e) Volume of Daily Cover
Vd= 0.1 x 3.118 x 10^6
= 0.3118 x 10^6 cu.m.
(f) Volume of Liner and Cover Systems
Vc= k x Vw (k=0.25 for 10m high landfill, k=0.125 for 20m high landfill)
=0.125 x 3.118x 10^6
= 0.39 x 10^6 cu.m.
Vs= m x Vw (m=0.10 for biodegrable waste)
=0.3118 x 10^6 cu.m.
(g) First Estimate of Landfill Volume Ci (Ci=Vw+Vd+Vc-Vs)
Ci= (3.118 + 0.3118 + 0.39 – 0.3118) x 10^6
= 3.508 x 10^6 cu.m.
(h) Likely Shape of Landfill Rectangular in plan (length : width = 2:1)
Primarily above ground level, partly below ground level.
(i) Area Restrictions : Nil
(j) Possible Maximum Landfill Height = 20 m
(k) Area Required = (3.508 x 10^6 )/20
= 1.754 x 10^5 sq.m.
(l) Approximate Plan Dimensions (length : width = 2:1)
=297m x 594m
2. Siting
Location of the landfill is an important factor in the design of a landfill. I took into
consideration the following factors before determining where the landfill is to be
sited:
1.
Soil
Soil should be of sufficiently low permeability to significantly slow the passage
of leachate from the site. Also the soil should be deep to provide soil for lining and
covering solid waste after each disposal day.
2. Slope
The slope of the land is key as it can reduce or increase the stability of the landfill.
Areas of lower slope should be considered as steep slopes are difficult to level and
also prone to erosion
3. Distance from protected area
A distance greater than 2500m from protected or zoned areas e.g forests should be
considered
4. Distance from roads
The landfill should be close to road networks for accessibility and ease of
transportation, consequently reducing relative costs.
5. Distance from rivers
In order to reduce vulnerability to ground and surface water pollution from
contamination, landfills should not be located near streams or rivers
6. Land use, availability and ownership
Landfills should not be placed too close to settlement areas and recreational centers.
Also, there should be adequate land area and volume to provide the landfill capacity
to meet the projected needs. Land ownership should not be conflicted. Preferably
government land should be used and if privately owned the land should be acquired
legally following the Lands Act.
Based on the above landfill site selection criteria, an agricultural land in Kiambu
subcounty was found to be suitable due to:
 Adequate land area and volume to provide sanitary landfill capacity to meet
projected needs.
 No environmentally significant wetlands of important biodiversity or
reproductive value are present within the potential area of the landfill cell
development.
 No known environmentally rare or endangered species breeding areas or
protected living areas are present within the site boundaries.
 No significant protected forests are within 500 meters of the landfill cell
development area
 No major lines of electrical transmission or other infrastructure (i.e., gas, sewer,
water lines) are crossing the landfill cell development area
 No underlying underground mines which could be adversely affected by surface
activities of land-filling, or minable resources, which could be rendered less
accessible by land-filling.
The site is isolated from populated areas and can be accessed through a murram road
which is about 11kms from the main Kiambu-Githunguri tarmac road.
3. Design
a) Waste cells
The cell is the basic building block of a sanitary landfill. They occupy the majority of
the project area.
The design of these cells will comply with the international standards in terms of
environmental protection. The main technologies employed in the design include:  State-of-the-art bottom liner systems;
 Modern leachate collection, treatment, landfill gas collection and monitoring;
 Groundwater monitoring;
 Engineered final cover layer.
Design of the waste cells will be from top to bottom. The vertical design will consist
of final cover that allows efficient landfill gas collection and prevents infiltration from
rain water, a bottom layer of a landfill consisting of drainage layer which allows
collection of leachate for further storage and treatment; and a barrier layer made of a
composite liner system, which acts as confinement between the waste deposit and the
substratum.
The drainage layer will consist of (from top to bottom):
 A geo-textile filter to prevent contamination or plugging of the drainage system
by solids;
 A layer of sand or gravel or a thick plastic mesh which collects leachate and
allows it to flow through the leachate collection system;
 Perforated pipes, specially designed for gravity flow of leachate towards low
points for further treatment.
The barrier layer will consist of (from top to bottom):
 A protective non-woven geo-textile to prevent punctures in the HDPE (High
density Polythene Sheets) sheet;
 A sheet of HDPE (at least 2 mm thick);
 A second woven geo-textile to protect the HDPE liner, (Sometimes, clay can be
added when available and when additional protection is required);
 The natural substratum prepared and graded according to the project profile.
b) Buffer zone
The buffer zone is an area of restricted activities, depending on the activity in adjacent
land uses. It also ensures a long term continuous availability of disposal sites by
avoiding potential conflicts between waste disposal sites and adjacent lands with
different users.
As required by the solid waste disposal guidelines, the landfill will have a protective
buffer zone of 250m all-round from the cell’s perimeter as shown below:
The buffer zone will be used as a stockpiling for excavated soil material and it will
include a protective dike with vegetation.
c) The leachate management system
Leachate is liquid generated from rainfall and the natural decomposition of waste that
is filtered through the landfill to a leachate collection system. The leachate collection
system's job is to direct the leachate to collection sumps so it can be properly removed
from the landfill. It prevents the waste from seeping into the ground water system.
Considering the fairly high level of rainfall (1200 mm per year) and high evaporation,
within the project area; the volume of leachate is expected to range between 4,000 and
5,000 m^3 per year. The design capacity for leachate collection pond is based on a 3month capacity, taking into account the most unfavorable seasons in February, March
and April with the highest potential rainfall. The capacity takes into account
cumulative leachate production, rainfall with no or little recirculation.
In a conservative approach and to prevent overflowing, a capacity of 5,000 m3 which
includes period with high rainfall events at the leachate treatment system is proposed.
Potential methods for leachate treatment include:
 Recirculation of leachate through the landfill
 Disposal off-site to sewer for treatment as an admixture with domestic sewage
and physical-chemical treatment
 Membrane filtration
 Reverse osmosis
 Anaerobic biological treatment
 Aerobic biological treatment
 Constructed wetlands
Factoring in the high technology, high cost and high operation / maintenance system
associated with reverse osmosis plant, an anaerobic/aerobic system combined with
planted constructed wetlands (PCW) is proposed.
The leachate system will comprise of equipment that is part of the whole project and
includes the leachate collection pond, suitable piping, other ponds and the PCW.
It will comprise of a leachate storage pond whose design capacity is based on 3
months of leachate production to accommodate for any excess production or
contingencies in the treatment process, and variations of the surface in operation. The
corresponding volume to be stored will be about 5000 cubic meters. On site treatment
methods will be employed whereby lagoons with impermeable liners at the bottom
will be constructed to encourage a first phase of anaerobic decomposition, followed
by aerobic decomposition. Full evaporation in the final storage lagoon is desirable and
the discharge of treated effluent will be directed to an artificial wetland for further
tertiary treatment. If full evaporation is not possible, recirculation of treated effluent
back to the landfill is recommended. Continuous monitoring of effluent quality will
be done as per NEMA effluent discharge regulations.
After final treatment in the maturation pond, the effluent will meet the requirements
of NEMA and can be used to water the vegetation in the buffer zone or be discharged
into the river after normal quality tests as required by national and international
standards regarding effluent discharge have been conducted. A percolation area will
be designated just next to the maturation ponds for excess water, which cannot be
utilized.
d ) Landfill Gas (LFG)
The solution proposed is simple and inexpensive. It consists of an enclosed flare that
encloses the flame in an insulated cylindrical shroud. Shrouded landfill gas flares
have exit temperatures of around 760°C or 14000 °F – well above the dioxin
formation range (which ends around 400°C or 752°F).
Maintenance is limited and monitoring may be automated. However, flaring provides
no opportunities for energy recovery and is included in the project for safety purposes
when dumping excess gas is required.
Possible landfill gas uses
Once the LFG gas is collected, the gas may be used in various processes including
energy recovery applications e.g:
 Flaring
 Heat applications- industrial boilers, space cooling and heating
 Processing and use in fuel cells
 Incorporation into local natural gas network.
4. Operation
The operation of the proposed sanitary landfill will be done as per the proposed
general operation of the landfill in the figure below. However, after unloading, sorting
will take place to separate the biodegradable waste from other wastes. The
biodegradable waste will be landfilled, then compaction followed by daily cover.