Members of Unite walked out on March 11, leading to rubbish piling up on the streets.
A rally in support of the strikers will be held in the city on September 20.
Unite general secretary Sharon Graham said the workers were “resolute” in their determination to continue taking industrial action.
She told the PA news agency that the dispute was affecting Unite’s relationship with the Labour Party and the Government.
Unite is facing legal action over the dispute and Ms Graham said if the union is fined, the money will be paid out of its Labour affiliation fund.
“Our members are resolute in their determination to win this dispute and are disgusted at the behaviour of the Labour council and the Labour Government.”
Unite says hundreds of its members face pay cuts of up to £8,000 a year, adding that the council has spent millions of pounds hiring agency workers to collect rubbish.
The union has warned the strike could continue into next spring.
The council insists it has to make changes to its refuse collection service to make it more efficient.