<Page>

                                                                    Exhibit 3.13

                                                                          PAGE 1

                                    DELAWARE

                                 THE FIRST STATE

     I, HARRIET SMITH WINDSOR, SECRETARY OF STATE OF THE STATE OF DELAWARE, DO
HEREBY CERTIFY THE ATTACHED ARE TRUE AND CORRECT COPIES OF ALL DOCUMENTS ON FILE
OF "CASELLA NH INVESTORS CO., LLC" AS RECEIVED AND FILED IN THIS OFFICE.

     THE FOLLOWING DOCUMENTS HAVE BEEN CERTIFIED:

     CERTIFICATE OF FORMATION, FILED THE SIXTH DAY OF SEPTEMBER, A.D. 2001, AT
12 O'CLOCK P.M.

     AND I DO HEREBY FURTHER CERTIFY THAT THE AFORESAID CERTIFICATES ARE THE
ONLY CERTIFICATES ON RECORD OF THE AFORESAID LIMITED LIABILITY COMPANY.


[SEAL]                                 /s/ Harriet Smith Windsor
                                      -----------------------------------------
                                      Harriet Smith Windsor, Secretary of State

3433259   8100H                         AUTHENTICATION: 1782738

020316418                                         DATE: 05-17-02

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                            CERTIFICATE OF FORMATION
                                       OF
                          CASELLA NH INVESTORS CO., LLC

          This Certificate of Formation of Casella NH Investors Co., LLC (the
"LLC"), dated as of August 31, 2001, is being duly executed and filed by KTI,
Inc., as an authorized person, to form a limited liability company under the
Delaware Limited Liability Company Act (6 DEL.C. 18-101, ET SEQ.).

          FIRST. The name of the limited liability company formed hereby is
Casella NH Investors Co., LLC.

          SECOND. The address of the registered office of the LLC in the State
of Delaware is c/o The Corporation Trust Company, 1209 Orange Street,
Wilmington, New Castle County, Delaware 19801.

          THIRD. The name and address of the registered agent for service of
process on the LLC in the State of Delaware is The Corporation Trust Company,
1209 Orange Street, Wilmington, New Castle County, Delaware 19801.

          FOURTH. KTI, Inc., a New Jersey corporation ("Member"), is the sole
member of the LLC.

          FIFTH. The LLC is member-managed and has no managers. Member acting
alone, and without the consent of any other person, is authorized to manage and
control the business and affairs of the LLC and to take any and all action in
the name and on behalf of the LLC. Without limiting the foregoing, Member as the
sole member of the LLC may delegate such of its rights and powers to manage and
control the business and affairs of the LLC to such officers with such titles,
duties and responsibilities as Member shall from time to time determine to be
necessary or advisable.

          IN WITNESS WHEREOF, the undersigned has executed this Certificate of
Formation as of the date first above written.

                                        KTI, INC., Authorized Person

                                        By /s/ James W. Bohlig
                                           --------------------------
                                           James W. Bohlig
                                           Vice President


                                                          STATE OF DELAWARE
                                                          SECRETARY OF STATE
                                                       DIVISION OF CORPORATIONS
                                                       FILED 12:00 PM 09/06/2001
                                                          010441105 - 3433259