Nine people have been arrested and four factories shut down indefinitely in Nairobi for failing to comply with waste disposal regulations.
The National Environment Management Authority (Nema) Tuesday morning cracked down on facilities disposing of their waste without pre-treating it.
The companies shut down are milk producer Daima, Sameer Agriculture and Livestock Ltd, Chloride Exide Kenya Ltd, Synresins Ltd and Ennsvalley Bakery Ltd.
Speaking during the crackdown, Nema Acting Director-General Mamo B. Memo said: "We will continue with this exercise to ensure that our rivers are clean and that the facilities responsible for harmful discharge are shut down. The standards are very clear."
Among those arrested yesterday were Daima Operations Manager Kenneth Kareithi, Chloride Exide General Manager Karanja Njoroge, Ennsvalley Bakery Quality Assurance Manager Sylvia Ndung'u, Synresins Chief Executive Mira Shah and Synresins Production Manager Michael Mungai.
The crackdown follows an expose by the Nation on the pollution of the Nairobi River, which was found to be loaded with harmful substances such as lead, copper and mercury.
"Nema has identified 122 discharge points along the Nairobi River," Mr Mamo said.
"From the inspection conducted, 50 per cent of the firms have complied. The ongoing inspection is to ensure the remaining companies comply with Water Quality Regulation 2006, or else they will be shut down and their officials prosecuted."
Nema has so far raided 53 Nairobi-based facilities that use wet processes in their operations. Eight have been shut down this week.
Mr Mamo said the crackdown will extend to waste discharge from households along the Nairobi River after the industrial crackdown.
The crackdown revealed that, while Daima has a treatment plant, it does not treat all its waste before discharging it into the sewer system or the environment.
Meanwhile, Synresins does not have a treatment plant, while Chloride Exide treats its waste at a separate facility.
But Nema Nairobi County Director of Environment Njoki Mukiri said all waste from industrial facilities should be pre-treated before being discharged.
Synresins and Chloride Exide, which both deal with industrial chemicals, were categorised as high-risk facilities.
Mr Njoroge, Chloride Exide general manager, however, defended their operations, saying they treat their waste with soda ash and send it to a holding tank within the premises where they store their effluent, which is then transported to a separate treatment facility in Athi River.
Synresins, for its part, said it does not discharge toxins into the environment or the sewer system but has the discharge collected by an exhauster every month.