The National Environmental Management Authority (Nema) has shut down 25 factories accused of discharging effluent into Nairobi River.
Nema Director-General Geoffrey Wahungu said on Wednesday that the factories illegally discharged waste into the severely polluted river that is now being cleaned.
Mr Wahungu added that the factories, which he did not name, did not meet Nema's standards and therefore could not be allowed to continue operating.
The authority earlier said three factories would be shut down on Wednesday as part of joint efforts with Nairobi County to clean up the river.
Giloil Industries, a cooking oil manufacturer, was the first to be shutdown, following accusations of failure to comply with regulations on discharging waste. Its workers were left stranded.
Mr Wahungu said the exercise will continue until all companies discharge their waste properly as it works with Nairobi to reclaim all of the county's rivers.
“We are currently dealing with industries that discharge untreated effluent into the environment," he said.
The pollution of Nairobi River and her tributaries results in the pollution of River Athi.
As the clean-up efforts continued, Nema said it had already closed 15 other factories for non-compliance.
Governor Mike Sonko supported the closure of the factories and warned that action will also be taken against hospitals which dump babies' bodies after abortions.
“We will not allow such factories to continue operating. I want to see them closed starting today. Clinics that dump the bodies of infants into the river will also be shut down,” said Mr Sonko.
Nairobi River's cleanup has been ongoing for more than a year now.
The exercise has seen long stretches of the river cleaned by the Ng’arisha Jiji team, which has recovered four bodies so far.
Nairobi's Environment minister Veska Kangogo said the illegal discharge of waste by factories and illegal dumping by residents were the main reasons for the river's pollution.
But she noted that there had been positive outcomes.
“We have started seeing the results of the exercise. Water is now flowing freely on many stretches of the river and kilometres of the bank have been reclaimed and turned into recreational parks,” the minister said.
Ngong, Mathare, Mbagathi, Kirichwa Kubwa and Kirichwa Ndogo are among the tributaries of the river that have so far been cleared of garbage.
In April, Ms Kangogo reported that the Nairobi River regeneration taskforce had identified companies and businesses responsible for pollution.
On Wednesday, she said City Hall will deploy enforcement officers along the most affected parts of the river to monitor those who illegally dump or discharge waste into it.