Waste Water Management
Sewage Treatment & Disposal
Strictly no water should be regarded as waste, even in wet countries water is a valuable resource, to be re-used whenever possible but humans cannot help polluting the water they use. We add dirt to it when we wash. More serious pollution is caused by our body’s waste products, namely urine and faeces which are carried away by water to form a polluted liquid called sewerage. In most places animal wastes and industrial effluents make the problems much complex.
However, the most effective and widely used treatment methods harness natural biological process to purify the sewerage or decompose the sludge’s produced. In this way, the impact of human waste on the environment can be minimised.
RATJOMOSE SEWAGE TREATMENT
The Ratjomose Sewage Treatment Plant is the largest sewage treatment facility in Lesotho . The Plant makes use of both the conventional and waste stabilization pond methods to treat or purify sewage. It is located on the South West of Maseru on the banks of the Mohokare ( Caledon ) River. It serves a collection area of 20 sq. kilometers, a fifth of the present inhabitated Maseru reserve area. Because of the topography of Maseru , 11 pumping stations have been installed to assist in transporting sewage to the treatment plant. The drainage areas are: Lesotho College of Education, Phahameng and BEDCO, Maseru East, Moshoeshoe II, Cathedral and Stadium Areas, Sea-Point, Maseru Central, Maseru West, Industrial and Hoohlo, Europa, Hillsview and Thetsane, including Thetsane Industrial Area. The population served is estimated to be over 12,000.
The sewage flows through two main sewers 1.5m dia., which are capable of handling a flow of 20, cubic metres per day, or 20 million litres of sewage everyday. The plant can handle an average dry weather flow of 10, 000 cubic metres per day, or 10 million litres per day. It is capable of handling peak flows of over 20 cubic meters per day, or 20 million litres per day.
On arrival at the works, large debris such as rags, maize cobs, etc are removed from the sewage by means of screening, through one bar screen with bars at 25mm spacing. The screenings are manually removed by rakes and are either dried and burned or are buried in trenches which are covered with a layer of soil at the end of each day.
Two grit channels 12m long parallel to each other are used alternatively to allow grit, sand and other mineral solids to settle, but the channels are designed in such a way that they allow other organic solids to pass with the sewage. The grit is then removed by shovels into wheelbarrows for burial in trenches and are also covered with a layer of soil.
Four circular primary sedimentation tanks are used in parallel to one another, to slow down effluent from the preliminary treatment in order to allow the suspended solids to settle prior to the next stage. The solids may be granular, salts or flocculants and the tanks are designed to facilitate all these to settle in harmony. 40% of the effluent from the preliminary treatment goes to the old primary sedimentation tanks (#19 on site plan), while the other 60% is diverted to the new primary sedimentation tanks (#17).
The settled matter, now called raw sludge, is collected from the tank floors into the sludge hoppers by means of scrapers, while the floating debris are removed by the skimmers into the scum boxes. Both operations are done by mechanical means and the effluent, now called settled sewage, flows to the next stage. The effluent from the old primary sedimentation tanks (PST’s) flows to the biological filters (#20), while effluent from the new PST’s flows directly to the pond feed chamber (#18).
Biological or percolating filters
These are also known as trickling filters or sprinkling filters and consist of beds of stones 1.8 deep, provided with a distribution system for applying settled sewage from the old PST’s. Living organisms on the filter media oxidize and purify the sewage, ending up with a by-product called humus. Four filters are simultaneously used at Ratjomose.
Humus Tanks or Secondary Sedimentation Tanks
The effluent from the percolating filters contains newly formed settleable sludge (humus) which again, by slowing down the flow, is settled in the tanks. The tanks are identical to the primary sedimentation tanks, as is the process, but with the retention time increased. The settled sludge is returned to the works inlet and the effluent which is aesthetically clean to the naked eye, then flows to the pond feed chamber.
Facultative ponds or Lagoons
The effluent from the humus tanks and effluent from the new PST’s converge at the pond feed chamber. Half of it flows to the first facultative pond (#1), and the other half flows to the second facultative pond (#2). Each facultative lagoon has two zones of treatment: an aerobic (oxygen-containing) surface layer and an anaerobic (without oxygen) bottom layer.
Suspended solids contained in the sewage will either be used as food by the aerobic bacteria or settle to the bottom or be digested by the anaerobic bacteria. Dissolved solids are also used by both types of bacteria. Oxygen for the aerobic bacteria is provided by the algae growing in the pond and by wind action. The quantity of organic matter used by bacteria is usually referred to as BOD or Biochemical Oxygen Demand.
The effluent from the first facultative pond bypasses the second facultative pond and flows to the first maturation pond (#3). Effluent from the second facultative pond also flows to the first maturation pond.
Maturation ponds are intended to serve two purposes:
Provide further reduction of BOD
Provide further reduction of bacteria found in faeces called faecal coliforms by means of ultraviolet radiation from the Sun.
Maturation ponds are operated in series. The effluent from the last maturation pond (#18) flows to the river. Presently, effluent from humus tanks bypass the 1st facultative pond to the 2 nd facultative pond then flows in series through maturation pond to the outlet.
Sludge Digestion Tanks
Two sludge digestion tanks are used in series, to stabilize the sludge by allowing the anaerobic micro-organisms to break down the organic matter present, thus rendering it more stable.
Digested Sludge Drying Beds
This is the only unit where actual filtration is taking place. Digested sludge from the digestion tanks is spread in thin layers in the beds and left to dry through filtration and evaporation. The filtrate is pumped back to the inlet works and the dried sludge is collected by individual farmers for recycling into agricultural land.
The guidelines for reuse are under development.
Digested Sludge Lagoons
The purpose of the lagoons is to hold the digested sludge for moisture reduction by evaporation and drainage of liquor to a holding tank. The liquor is pumped back to the inlet works.
VIP Contents Lagoons
The purpose of the VIP contents lagoons is to hold VIP contents for further digestion and drying.
The pump room has:
-2 pumps for pumping raw sludge from the PST’s to the sludge digesters.
-2 pumps for pumping:-
a) humus or sludge from secondary sedimentary tanks;
b) drying beds filtrate; and
c) liquor from digested sludge and VIP lagoons
Summary and other facts about the plant
Population served: those using VIPs = 60 000, those using conservancy tanks= 4,940, those connected to sewers= 12,000. Collection areas served: more than 20 sq.km Current average flow o works: 7.0 million litres per day:-
a) One screen
b) Two grit channels
c) Four pst’s
d) Four biological filters
e) Two secondary sedimentation tanks
f) Two facultative ponds
g) Six maturation ponds
h) Two sludge digestion ponds
i) Forty eight sludge drying beds
j) Four sludge lagoons
k) Two VIP lagoons
l) One pump room with four pumps
Staff employed = 5
Every week samples taken at different stages of the Plant are sent to the Central Laboratory for analysis.
The sewerage network
Householders are responsible for the maintenance and repair of their drains until they connect with the public sewer. Our sewers are regularly inspected and maintained to ensure they operate as effectively as possible.
Public and private sewers
Public sewers are those sewers that are owned by WASCO which is also responsible for their maintenance (all lateral sewers collecting sewage from private properties to sewage pumping stations and waste water treatment plant), while homeowners are responsible for private sewers (from the premises into the public sewer man hole) and drains.
When a public sewer gets blocked, WASCO will unblock the sewer without charge. When a private sewer or drain gets blocked, the property owner is responsible for unblocking it. He may hire a private plumber or ask WASCO to unblock it, but has to pay for this service.
Disposal of unwanted material
The drains connecting your home to the main sewer are designed in such a way that they only carry water, toilet paper and human waste not any other items bigger than that size. However, most people use sewers as garbage disposal units. As a result a number of people cause blockages in sewers which is not user-friendly to the environment.
It should be noted that some items though called disposable are not safe to flush down the toilet. Instead you should dispose them into a bag or bin, but don’t flush them. This will save unnecessary costs or expenses as plumbing bills may hinder some of your projects.
What to do with disposables?
All disposable products should be put in a bin or bag and not to be flushed down the toilet as they can find their way into the environment.
The following items should be disposed off in the ways recommended:-
Cotton buds, bandages, plasters and dental floss – dispose them in the bin.
Sanitary towels and pads – use the special bags for sanitary protection available from pharmacies and dispose off in rubbish bins.
Disposable nappies wrap them and dispose in the bin or use nappy bags which are available from various supermarkets.
Razors and blades – put these into a rigid container before placing in the bin.
Plastics bags and newspapers place them in the bin.
Causes of sewer blockages?
WASCO sewers are designed to take away sewage from properties. However, some people put garbage in the toilet and this causes blockages.
Who is responsible for Sewage overflow?
WASCO is responsible for sewage overflow if caused by a blockage in a public sewer and has to ensure that it is cleared.
If the overflow is caused by any other problem in the customers’ private sewer or drain, the owner of the property is responsible dealing with the flooding. In a case where a sewer serves more than one property, WASCO is responsible. Who to contact and what to do if there is a blockage within your property – Contact sewage division immediately if there is a blockage within your property. Telephone: 22312213/22312449/22313943 or our toll free number 800 22 011.
Take note of the following precautions to protect you and your family if there is a blockage:
Wear protective clothing and protect any cuts.
Dispose off any vegetables or fruit growing in your garden if it is flooded or use a hose pipe to clean it.
Keep children away from flooded areas for a couple of days until such time the area dries up.
Consult your doctor or physician if you suffer vomiting or diarrhoea following a flooding incident.