$this->_redirect('*/*/', array('_query', array('category_id',$category_id));
Should do the job, see also: http://stackoverflow.com/a/4540289/622945https://stackoverflow.com/a/4540289/622945 and your duplicated question on Stack Overflow: http://stackoverflow.com/q/33435543/622945https://stackoverflow.com/q/33435543/622945