Can not populate JQuery DataTable with my json data

I'm stuck completely while accessing the data from asp.net web service to .aspx page. I used the Newtonsoft.Json.JsonConvert.SerializeObject(qry, Formatting.Indented) for creating a JSON string.

This library provides a genuine and valid JSON string like following:
[ { "DepartmentId": 0, "Department": "CIVIL ENGG.", "DepartmentShortName": "CIVIL" },
{ "DepartmentId": 10, "Department": "COMPUTER SCIENCE ENGINEERING", "DepartmentShortName": "C.S.E." },
{ "DepartmentId": 21, "Department": "ELECTRICAL & ELECTRONICS ENGG.", "DepartmentShortName": "E.N." },
{ "DepartmentId": 31, "Department": "ELECTRONICS & COMMUNICATION ENGG.", "DepartmentShortName": "E.C.E" } ]

The code snippet to populate the datatable is:
$(document).ready(function() {
$.ajax({
type: "POST",
url: webServiceURL,
data: "{}",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function(json) {
$('#grid').dataTable({
"bProcessing": true,
"bServerSide": true,
"sAjaxSource": webServiceURL,
"fnServerData": fnDataTablesPipeline
});
}
});
});
for complete set of code, I referred the http://www.datatables.net/examples/server_side/pipeline.html link and indeed tried many other options but no luck.

I suspect that it can be occurred because of 'sEcho', 'iDisplayStart' and 'iDisplayLength' are missing from json but when I used my json string within aspx page and passed to 'aaData', it works fine.

I'm lost and not finding any solution since last three days. I believe some help will come out here..

Thanks in advance