trying to use batch_update()

Here it looks like you wrote a SELECT query, did not utilize the result, and then wrote a separate UPDATE query with no conditions.

I think you're wanting to do something like this..

PHP Code:

$this->db
    ->set(['in_population' => 1])
    ->where('email', $_SESSION['userid'])
    ->where('campaign', $_SESSION['campaign'])
    ->group_start()
        ->where('employee_title',  $exclude_string)
        ->or_where('job_code',  $exclude_string)
        ->or_where('department',  $exclude_string)
        ->or_where('role',  $exclude_string)
    ->group_end()
    ->update('employees');