Water Quality
Our focus is to always ensure delivery of quality water to the city

Waste water treatment

Introduction

Water is life!

As the saying goes water and clean environment is the key to the survival and existence of every living organism under the sun. For along time people have assumed that water as resource is free and unlimited. The bitter reality is water as a basic natural resource is becoming scarce each passing second due to mismanagement and pollution of the few natural catchments in the country and by extension the whole world at large.
 
From the current water crisis in Nairobi city and its environs people have come to accept the bitter truth that water is “gold so precious but hard to come by” any government has a role to play in making sure that its citizens access clean water and a health environment. The government of Kenya has for along time, been involved in the sourcing and distribution of this vital commodity to the Nairobi city residents and industries and also in waste water treatment through the city council of Nairobi. Due to challenges that were facing the Nairobi city council, its capacity to cater for its customers water requirements were minimal this prompted the government of Kenya to privatize the delivery of these services hence Nairobi city water and sewerage company Ltd was awarded the contract of sourcing, storage, treatment and distribution of water to the city residents and its environs
 
Nairobi Water Company was launched on 25th August 2005 as a private company for supplying drinking water and provision of sewerage services to the city of Nairobi.
 
NCWS Company has moved an extra mile to ensure environment is taken care of by improving and maintaining its waste water treatments facility.
All water supplied to Nairobians for domestic and industrial use turns to wastewater which is collected through the sewerage system to main sewer truck which finally deliver the water to the treatment plants.
The formation of environment department is to improve environmental measures on domestic and industrial waste load to sewer truck and finally discharge after treatment process to the natural environment. Final effluent water is discharged to the nearest river for reuse.

Waste water treatment:
Dandora estate sewerage treatment works is a section of Nairobi water and Sewerage Company.
Located at 26 km from Nairobi city along Kangundo road.
It treats about 80,0000m3/day equivalent to about 80% of waste water generated from Nairobi city.
 
Waste water is treated through two processes;
  1. Physical treatment
  2. Biological treatment.
 
These comprise of;
  1. Mechanical treatment facility
  2. Waste stabilization ponds, i.e.
    • Anaerobic ponds
    • Facultative ponds
    • And maturation, 1, 2 and 3 ponds
 
At the physical treatment facility, this is where large organic solids are physically removed both manually and mechanically at the four stages.
  1. course bar screens
  2. cup screens
  3. Grit trap and classifiers.

After it flows to the biological ponds where it's retained as per design for a period of:

  1. 5 -10 days---- anaerobic pond
  2. 35 days---facultative pond
  3. 5 days--- each maturation pond
 
Giving an approximate retention period of between 60-90 days for each treatment series.
 
All these ponds have been constructed in away that aid the application of biological treatment effectively.
For example the anaerobic ponds have an average depth of 4.7m and also are narrow (around 80m x 80m) with scum covering the surface which aid in creating anaerobic conditions hence activation of anaerobic bacteria to work on the organic load.
In the facultative ponds with approximate length of 700m x 300m wide and a depth of 1.7m this creates a favorable condition for the activation of facultative bacteria both anaerobic and aerobic conditions are found at this type of pond.
At the bottom we have anaerobic bacteria at the middle there is the facultative bacteria and at the surface there is the aerobic bacteria. In this pond also the amount of scum is minimal as compared to the anaerobic pond. Up to this point majority of organisms have been eliminated such as helminth eggs and larvae.
Then water is let into the maturation ponds with an average depth of 0.5 m and 300m x 300m. At this depth sunlight is able to penetrate to the bottom hence killing the pathogens by use of UV rays.
Within these ponds various naturally occurring biological micro-organisms break down the organic particles to carbon dioxide and water.
Other non bio-degradable particles precipitate at the bottom of these ponds as part of sludge.
 At DESTW two species of fish have been introduced in maturation three (catfish and tilapia) to check on the effectiveness of the wastewater treatment as the fish are delicate living organisms which can succumb to slight pollution in the water.
Also algae are used to asses the extent of industrial pollution in the ponds usually when pinkish to red algae appear on the surface is an indication of excessive industrial pollution. Normally various ponds exhibit different water Colour this is due to the species of algae abundant at that pond.
 
The treated wastewater is released back to the natural environment. The quality of the wastewater is controlled and monitored at every stage to ensure that what is discharged to Nairobi River meets the required standards as set by NEMA.
 
Challenges facing wastewater treatment in DESTW

Dumping of overloaded effluent into the sewer system which does not meet the required discharge standards.
 
These include among others;
  1. Non-biodegradable detergents
     
  2. Plastics and polythene carrier bags
     
  3. Large physical materials (hides and skins)
     
  4. Oils, fat and grease
     
  5. Non-biodegradable solids eg heavy metals, sand and grit
     
  6. High levels of nitrates and phosphates
 
 Impacts of the above challenges
 
  1. Regular breakdown of machines and equipments at the mechanical treatment facility due to overloading,
     
  2. Nuisance presence of plastics and other floating materials in the biological ponds,
     
  3. Reduced effectiveness of the pond performance due to these floating materials,
     
  4. Proliferation of different species of algae growth and other water weeds,
     
  5. Reduced performance of the biological micro-organisms due to presence of heavy metals, oils and grease,
     
  6. Reduced lifespan of the biological ponds due to faster build up of inorganic solids,
     
  7. Poor final quality of effluent due to high algae growth.

Overall outcome

  1. Algae growing in any aerated systems, ponds or wastewater treatment plants increase both the TSS and the BODs of the effluent. 
     
  2. This cause false high BOD and TSS levels in final effluent permit.
     
  3. Excess algae can also create nuisance surface scum, poor water clarity and noxious odors. 
     
  4. Excessive plant growth can physically prevent mixing of oxygen through the water; plant dies off and fish  kill can occur with too much algae. 
     
  5. Under certain conditions, especially when oxygen is absent from the bottom waters, phosphorous is released from bottom sediments into the overlying water. 
     
  6. Algae production is many times related to the levels and ratios of nitrogen (N) and phosphorous (P) in water.
     
  7. Also it’s many times due to an excess of BOD levels. This is because the carbon often is a food source for some of the cyanobacteria and algae that grow.
     
  8. In turn algae clouds water clarity and decreases the depth of light penetration. This harbors the effect of removal of pathogens using UV rays from the natural light.
 


DANDORA SEWERAGE STABILIZATION PONDS (RUAI):

Layout of the ponds:


(i) Old series –Series 1 & 2    (ii) New series –Series 3 to 8

Key:
       A- Anaerobic Pond
       F-Facultative ponds
       M-Maturation ponds

Features at the ponds:

i.   Birds (more than 200 species
ii. Wild water weeds
iii. Algae
iv. Wild animals i.e. Crocodiles, Hippos, Tortoise & Alligators
v.   Fish e.g. Catfish & Tilapia

N/B
The stabilization ponds are the second biggest in Africa and covers 200 hectares of land with over 500 hectares for future expansion.

© 2011. Nairobi City Water and Sewerage Company Limited. All rights reserved.
ISO-9001 Certified