var ms = new MemoryStream();
var sw = new StreamWriter(ms, Encoding.UTF8);

...

//XDocument
response.Save(sw);

            if (null != WebOperationContext.Current)
            {
                WebOperationContext.Current.OutgoingResponse.ContentType = string.Format("text/xml; charset={0}", Config.Encoding);
                WebOperationContext.Current.OutgoingResponse.Headers.Add(HttpResponseHeader.CacheControl, "no-cache");
            }

	        sw.Flush();
            ms.Position = 0;

            return ms;

if (null != WebOperationContext.Current)
            {
                WebOperationContext.Current.OutgoingResponse.ContentType = string.Format("text/xml; charset={0}", Config.Encoding);
                WebOperationContext.Current.OutgoingResponse.Headers.Add(HttpResponseHeader.CacheControl, "no-cache");
            }

            return new MemoryStream(EncodeResult(response));
........

private static byte[] EncodeResult(XDocument xDocument)
        {
            return Encoding.GetEncoding(Config.Encoding).GetBytes(string.Concat(xDocument.Declaration, xDocument));
        }