The incident occurred in Venkateshwara Hatcheries in Moram village of Palamaneru block in the district.
The workers were cleaning the septic tank in which the waste gets deposited. Four workers who had first entered the manhole fell unconscious. Three workers who tried to save them were also taken ill.
The villagers pulled all of them out by removing the ceiling of the tank. While four of them died on the way to hospital, the remaining three succumbed at the hospital.
Officials said the workers died of asphyxiation or due to inhaling of poisonous gas. The district officials have ordered a probe into the incident.
The company has announced Rs 15-lakh compensation each to the families of the deceased. The state government has also announced a compensation of Rs 10 lakh each.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.