Manufacture of plastic bags is still continuing more three months after the ban of the products.
The underground manufacturers are mainly based in Nairobi's Industrial Area, Mlolongo in Machakos County and Thika, and they are feeding traders who are using plastic bags for packaging.
Ban on plastic carrier bags came into force in August 28, which means that anyone found selling, manufacturing or carrying them could face fines of up to Sh4 million or prison sentences of up to four years.
National Environment Management Authority (Nema) said it will intensify surveillance following intelligence of increased illegal activities by manufacturers.
"We are following intelligence of illegal manufacturing. Two manufacturers have already been arrested," said Mwanzei Ali, deputy director at Nema.
Sources indicate that nearly 20 plastic bag manufactures are operating illegally in Nairobi, highlighting the weakness of implementing the ban as traders continue to defy the directive.
Nema said 20 people were arrested in Bomet, Nyeri and Mombasa counties in a crackdown that has mainly focused on traders and users of plastic bags.
A woman in Nyeri was released from court on Sh2 million bond after being arrested with 17 different types of the banned plastic bags.
Manufacturers who use polythene to wrap products are exempted from the ban.
Industrialists lobby Kenya Association of Manufacturers (KAM) said the ban will cost 60,000 jobs and force 176 manufacturers to close shop. Before the ban Kenya was also a major exporter of plastic bags to the region.
It joined more than 40 other countries including China, France, Rwanda, and Italy that have banned, partly banned or taxed single use plastic bags.
It took Kenya three attempts over 10 years to finally pass the ban. The government reckons that environmental concerns were more important than commercial interests.
Some researchers say a tax or a ban on plastics is not the long- term solution to solve pollution problems, arguing that it would deprive the world of a cheap and valuable material.
They argue the solution is to make the bags biodegradable and recyclable.
A Nairobi-based plastic manufacturing company has threatened to sue the State for effecting the ban that has seen it run into losses amounting to at least Sh2 billion.
Hi-plast Limited wants the Attorney-General to issue guidance on whether ministers can issue a far-reaching decision like the ban on plastics without public participation and input of Parliament.