Less than half of South Africa's 821 sewage works are treating the billions of litres of effluent they receive each day to safe and acceptable standards, according to the latest Green Drop Report.

The report – a measure of the state of wastewater treatment plants in all nine provinces – was released by Water and Environmental Affairs Minister Edna Molewa on Thursday.

While it awards Green Drop status to 40 plants – up from 33 in 2009 – it warns that another 460 plants (56% are either in a “critical state” or delivering a "very poor performance".

The latest report examines wastewater treatment at 821 plants in 156 municipalities – the previous (2009) report examined 444 plants in 98 municipalities – and says this is "100% coverage of all systems".

It is understood the report does not cover treatment works owned by public works, such as those at prisons, and other private operators.

Many of the poorly performing plants are located in the country's poorer provinces, including the Eastern Cape, Free State, Northern Cape and Limpopo.

"The Western Cape, followed by KwaZulu-Natal and Gauteng, are producing the high-performing waste water systems; Eastern Cape, followed by Free State, Northern Cape and Limpopo, are producing the bulk of the systems that are in critical and poor-performing positions."

The report's findings imply that millions of litres of untreated or inadequately treated sewage are being illegally discharged into rivers and streams each day, mainly by small town municipalities.

In a national overview, the document describes the overall "risk trend" of the 821 treatment plants as "neutral to negative".

It says that compared to 2008, the number of "critical risk" waste water treatment plants has increased from 129 to 137; and the number of "high risk" plants from 264 to 284.

At the other end of the scale, the number of plants previously deemed low risk" has dropped – from 196 in 2008, to 138 now.

The report says municipal waste water treatment service performance around the country varies from "excellent" to "unacceptable".

"Analysis of the Green Drop results indicates a fairly good national score of 71%. However, this value might be skewed, as a few excellent provincial scores would balance out the lower provincial performers."

The number of plants achieving a Green Drop score of more than 50% has decreased proportionally, from 49% in 2009, to 44%.

"This trend can possibly be explained by considering that 377 'first time' systems were assessed and many of these achieved low Green Drop scores."

The 821 sewage plants around the country treat a total flow of about 5,258 billion litres of waste water a day, almost half of it in Gauteng.

"Analysis of the operational flows indicates that Gauteng manages the bulk of the national load (49%), followed by the Western Cape (17%) and KwaZulu-Natal (14%).

"The balance of the provinces receive and treat the remaining 20% of waste water generated in South Africa."

The report says that of the 821 plants, a total of 40 – those awarded Green Drop status – were in an "excellent situation", 78 were "good”, and 243 delivered an "average performance".

However, a total of 143 delivered a "very poor performance"; and 317 were in a "critical state".

The report's release comes at the end of a three-day municipal water quality conference in Cape Town.

The names of the municipalities awarded Green Drop status will be announced at an awards dinner in Cape Town later on Thursday.