Cara menambahkan variable pada session

Bagiamana cara menambahkan variable array pada session? jadi disini session hanya berisi

array(2) { ["__ci_last_regenerate"]=> int(1569293976) ["tugas"]=> string(5) "users" }

nah saya mau menambahkan 'department' setelah "users" pada array diatas, baimana caranya?

public function login()
	{
		$ceks = $this->session->userdata('tugas');
		if (isset($ceks)) {
			redirect('');
		} else {
			$this->load->view('web/header');
			$this->load->view('web/login');
			$this->load->view('web/footer');

			if (isset($_POST['btnlogin'])) {
				$username = htmlentities(strip_tags($_POST['username']));
				$pass	   = htmlentities(strip_tags(md5($_POST['password'])));
				$dept	   = htmlentities(strip_tags($_POST['dept']));

				$query  = $this->Mcrud->get_data_by_pk('tbl_user', 'username', $username);
				$cek    = $query->result();
				$cekun  = $cek[0]->username;
				$jumlah = $query->num_rows();

				if ($jumlah == 0) {
					$this->session->set_flashdata(
						'msg',
						'
									 <div class="alert alert-danger alert-dismissible" role="alert">
									 		<button type="button" class="close" data-dismiss="alert" aria-label="Close">
												<span aria-hidden="true">×  </span>
											</button>
											<strong>Maaf "' . $username . '"</strong>tidak memiliki hak akses.
									 </div>'
					);
					redirect('web/login');
				} else {
					$row = $query->row();
					$cekpass = $row->password;
					if ($cekpass <> $pass) {
						$this->session->set_flashdata(
							'msg',
							'<div class="alert alert-warning alert-dismissible" role="alert">
													 		<button type="button" class="close" data-dismiss="alert" aria-label="Close">
																<span aria-hidden="true">×  </span>
															</button>
															<strong>Username atau Password Salah!</strong>.
													 </div>'
						);
						redirect('web/login');
					} else {

						//
						$row = $query->row();
						$cekdept = $row->dept;
						if ($cekdept <> $dept) {
							$this->session->set_flashdata(
								'msg',
								'<div class="alert alert-warning alert-dismissible" role="alert">
													 		<button type="button" class="close" data-dismiss="alert" aria-label="Close">
																<span aria-hidden="true">×  </span>
															</button>
															<strong>Department Salah!</strong>.
													 </div>'
							);
							redirect('web/login');

						} else {
							//


							$this->session->set_userdata('tugas',"$cekun");
							redirect('web');
						}
					}
				}
			}
		}
	}