Victims of Hungary’s worst ever toxic spill, which killed 10 people and injured 150 in 2010, voiced outrage after the boss of the alumina plant that caused the disaster was cleared of any wrongdoing.

Zoltán Bakonyi, the former director of the MAL plant in Ajka, and 14 employees were acquitted of charges of negligence, waste management violations and damages to the environment.

Prosecutors had demanded prison sentences for all those on trial over the disaster, which had sent toxic sludge cascading into villages in western Hungary after the plant’s holding reservoir burst its walls on 4 October 2010. But the court said the employees – which also included several senior managers – could not be held criminally responsible because the disaster had ostensibly been caused by a “loss of stability in the undersoil”.

The judgment sparked strong reactions in the packed courtroom in the city of Veszprém, with one man shouting “Outrageous verdict! We will protest!” before being escorted out by a security guard. The man had also unfolded a banner showing pictures of destroyed homes, with the words: “This is all 10 people’s lives are worth?”

A 54-year-old metalworker who lost his parents in the disaster shook his head at the ruling, saying he would “never get over what happened”. “The body of my father was only found a week after the accident, covered in mud on a football pitch, some 5km from his home,” Gyula Tokolics said. “I discovered my mother’s body in the house. She had just served lunch.”

The catastrophe unfolded when the reservoir cracked open after weeks of heavy rain, releasing 1.1m cubic metres of poisonous red sludge. The mud – a caustic byproduct of aluminium extraction – rushed into the nearby villages of Kolontár, Devecser and Somlóvásárhely.

Flows of 2 metres toppled cars and submerged homes and businesses. Many of the survivors suffered chemical burns. The sludge also wiped out almost all life in nearby rivers and even spread to the Danube. In total, the devastation spread across an area of more than 15 sq miles.

The damaged reservoir seen from the air. When its walls cracked, 1.1 million cubic metres of poisonous red sludge was released. Photograph: Laszlo Balogh/Reuters

Responding to the disaster, the Hungarian government declared a state of emergency and evacuated about 8,000 people. Over the following months, workers toiled to remove the mud from the flood plain and doused the area with acid.

Authorities imposed a 135bn forint (£330m) fine on MAL in 2011 and moved to nationalise the plant. Last February, Budapest set up a compensation fund for the victims, with many claims still outstanding.

But despite vast sums spent on depolluting the region, it still bears traces of the accident. Hundreds of hectares of land remain sealed off and cannot be used for cultivation. Dozens of local residents moved elsewhere in the aftermath. In Devecser, church bells ring every year on the anniversary of the spill. The disaster was bound to leave “an indelible mark”, the village’s then mayor, Tamás Toldi, had said in 2011.

Greenpeace lamented the lack of a firm ruling. “We are not necessarily saying that all 15 were guilty but we very disappointed that more than five years after the disaster there is still no one held responsible,” its spokesman Gergely Simon said.

“If you look at the scientific and technical evidence, we believe it is clear that the disaster was not a natural disaster but was due to human error,” he said. “Satellite images show that the walls of the dam were moving, and nobody checked the stability of the dam however between 2000 and 2010.”