<PAGE>   1
   
                                                                     EXHIBIT 4.6
    


               ORGANIZED UNDER THE LAWS OF THE STATE OF DELAWARE


NUMBER                                                                SHARES
   
                                                                   **        **
    


SEE RESTRICTIVE LEGEND(S) ON BACK


                           SYNAGRO TECHNOLOGIES, INC.
                            SERIES B PREFERRED STOCK
                                PAR VALUE $.002



   
     THIS CERTIFIES THAT (SPECIMEN) is the owner of
** _________________________________________________________________________ **
fully paid and non-assessable Shares OF THE SERIES B PREFERRED STOCK OF SYNAGRO
TECHNOLOGIES, INC. transferable only on the books of the Corporation by the
holder hereof in person or by duly authorized Attorney upon surrender of this
Certificate properly endorsed.
    

     IN WITNESS WHEREOF, the said Corporation has caused this Certificate to be
signed by its duly authorized officers and to be sealed with the Seal of the
Corporation.

   
Dated  
      ----------------
    


   
/s/                                     /s/ 
----------------------------            ---------------------------
       SECRETARY                                  PRESIDENT
    



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The securities evidenced hereby have not been registered under the Securities
Act of 1933, as amended (the "Act"), and may not be transferred (other than
pursuant to Rule 144 or any similar analogous rule or rules) except pursuant to
an effective registration statement under the Act or in a transaction which, in
the opinion of counsel reasonably satisfactory to the Company, qualifies as an
exempt transaction under the Act and the rules and regulation promulgated
thereunder. In addition, the securities evidenced hereby are subject to the
provisions of a Registration Rights Agreement dated March 31, 1998, between the
Company and the original holder of such securities, a copy of which will be
furnished by the Company to the owner hereof without charge upon written
request. 

     The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations. Additional abbreviations may also
be used though not in the list.


     TEN COM   - as tenants in common   

     TEN ENT   - as tenants by the entireties
     
     JT TEN    - as joint tenants with right of survivorship 
                 and not as tenants in common

     UNIF GIFT MIN ACT   -  Custodian         (Minor)
       under Uniform Gifts to Minors Act      (State)

For value received, the undersigned hereby sells, assigns and transfers unto

                                                       PLEASE INSERT SOCIAL 
                                                         SECURITY OR OTHER 
                                                       IDENTIFYING NUMBER OF 
                                                              ASSIGNEE
                                                        [                 ]
------------------------------------------------------- 
 PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS OF ASSIGNEE





----------------------------------------------------------------------------

                                                              SHARES
------------------------------------------------------------ 

represents by the within Certificate, and hereby irrevocably constitutes and
appoints ______________ Attorney to transfer the said Shares on the books of
the within-named Corporation with full power of substitution in the premises.


Dated,
      ---------------------------



                                                  --------------------------
          In presence of


---------------------------------

NOTICE: The signature to this assignment must correspond with the name as
written upon the face of the certificate in every particular without alteration
or enlargement, or any change whatever.