You can change your code to this:
$where_au = "(library.available_until >= '{date('Y-m-d H:i:s)}' OR library.available_until = '00-00-00 00:00:00')";
$this->db
->select('*')
->from('library')
->where('library.rating >=', $form['slider'])
->where('library.votes >=', '1000')
->where('library.language !=', 'German')
->where($where_au)
->where('library.release_year >=', $year_start)
->where('library.release_year <=', $year_end)
->join('rating_repo', 'library.id = rating_repo.id');
Tip: to watch the generated query you can use
echo $this->db->last_query(); die();