<PAGE>   1

                                                                   EXHIBIT 10.15


                     FIRST AMENDMENT TO BFI RETIREMENT PLAN


         This First Amendment made as of the 31st day of March, 1995, by
Browning-Ferris Industries, Inc., a corporation organized and existing under the
laws of the State of Delaware (hereinafter called the "Primary Sponsor");


                                   WITNESSETH:

         WHEREAS, the Primary Sponsor maintains the BFI Retirement Plan under an
indenture dated December 30, 1994 (the "Plan"); and

         WHEREAS, the Primary Sponsor desires to amend the Plan to make minor
technical changes;

         NOW, THEREFORE, the Primary Sponsor hereby amends the Plan, effective
as if included in the December 30, 1994 Plan document, as follows:

                  1.By replacing Section 1.1 with the following:

                           "Account" means a hypothetical account balance
                  established for each Participant which equals the present
                  value Actuarial Equivalent of the Participant's Old Plan
                  Benefit, plus the total amount credited on behalf of the
                  Participant after October 1, 1993. The amount credited for
                  each Plan year shall equal the sum of:

                                    (a) 4.5 % of Compensation for October 1,
                           1993 through December 31, 1993, and thereafter, 4.5%
                           of Compensation for the Plan Year, of a Participant
                           who is an Eligible Employee (and with respect to any
                           temporary and seasonal Employee only, has received a
                           year of Credited Service for such Plan Year.) (In the
                           case of Employees who are employed as of the last day
                           of the Plan Year, such amount shall be credited as of
                           the last day of each Plan Year. In the case of
                           Employees who have a Severance Date during a Plan
                           Year, such Compensation shall be credited as of the
                           last day of the month following the Severance Date.);
                           plus

                                    (b) interest on the period's beginning
                           balance computed by multiplying such balance by 6%
                           per annum for 1993 and 1994, and thereafter, an
                           annual rate determined by the Committee prior to the
                           first day of each Plan Year, but not less than 4% and
                           not more than 12% per annum thereafter. (In the case
                           of Employees who are employed on the last day of the
                           Plan Year, such interest shall be credited as of the
                           last day of each Plan Year. In the case of Employees
                           who have a Severance Date during a Plan Year, such
                           interest shall be credited as of the last day of




<PAGE>   2


                           the month following the Severance Date, and as of the
                           last day of each Plan Year thereafter preceding the
                           Plan Year in which payment commences.) In either
                           event, such interest shall be credited for each Plan
                           Year through the end of the month coinciding with or
                           immediately preceding the date as of which payment
                           commences under the Plan.

                  Notwithstanding Subsection (a) above:

                           (1) a Participant who would otherwise be excluded
                  from receiving a credit for a period under Subsection (a)
                  above solely because the Employee was included in a unit of
                  Employees covered under a collective bargaining agreement
                  between a union and a Plan Sponsor shall receive a credit
                  under Subsection (a) for such period if the Participant is not
                  included in such a unit, and is an Eligible Employee, on his
                  Severance Date; and

                           (2) a Participant who reaches a Disability Retirement
                  Date shall receive a credit under Subsection (a) above equal
                  to 4.5% of the greater of the Participant's Compensation in
                  the Plan Year in which the Disability Retirement Date occurred
                  or the first Plan Year immediately preceding that date,
                  continuing for each Plan Year until the earliest of the date
                  as of which the Participant (1) reaches age 65, (2) is no
                  longer subject to a Disability, or (3) elects to begin
                  receiving payment under the Plan; and

                           (3) a Participant who has not incurred a Severance
                  Date and who is on an authorized medical leave of absence
                  shall receive a credit under Subsection (a) above for the Plan
                  Year in which the authorized leave started equal to 4.5% of
                  the greater of the Participant's Compensation in the Plan Year
                  in which the leave of absence started or the first Plan Year
                  immediately preceding that date."

                  2. By replacing the reference in Plan Section 1.3(b) to
         "Section 1.2(a)" with "Section 1.2".

                  3. By adding the following to Section 1.13(a)

                           "(1) if an Employee quits, retires, or is discharged
                  and then performs an Hour of Service within twelve months of
                  his Severance Date, then such period of severance shall be
                  taken into account in calculating Credited Service;

                           (2) if an Employee quits, is discharged or retires
                  during an absence from service of twelve months or less for
                  any reason other than quit, discharge or attainment of a
                  Retirement Date and the Employee then performs an Hour of
                  Service within twelve months of the date the Employee was
                  first absent from service, then such period of absence shall
                  be taken into account in calculating Credited Service;


                                      -2-

<PAGE>   3

                           (3) in the case of an Employee who remains absent
                  from service beyond the first anniversary of the commencement
                  of a period of absence (1) by reason of the pregnancy of the
                  Employee, (2) by reason of the birth of a child of the
                  Employee, (3) by reason of the placement of a child with the
                  Employee in connection with the adoption of the child by the
                  Employee, or (4) for purposes of caring for such child for a
                  period immediately following its birth or placement, the
                  period between the first and second anniversaries of such
                  period of absence shall not be counted as Credited Service. H

               4. By replacing Section 1.37(b) with the following:

                           "(b) for purposes of applying the Old Plan Benefit
                  Formula, each Section 401(a)(17) Employee's accrued benefit
                  will be the greater of the amount determined under Paragraphs
                  (1) or (2) below.

                                    (1) equals the accrued benefit determined
                           under the Old Plan Benefit Formula as applied to the
                           Employee's total years of Credited Service (to the
                           extent recognized for computing a benefit under the
                           Plan), and on the Employee's Compensation not in
                           excess of the applicable Compensation Limit in effect
                           on and after January 1, 1994.

                                    (2) equals the sum of (A) plus (B) below.

                                        (A) equals the greater of (i) or (ii)
                                     below.

                                            (i) equals the accrued benefit
                                         determined under the Old Plan Benefit
                                         Formula as applied to the Employee's
                                         total years of Credited Service before
                                         January 1, 1994 (to the extent
                                         recognized for computing a benefit
                                         under the Plan) and on the Employee's
                                         Compensation not in excess of the
                                         applicable Compensation Limit in effect
                                         before January 1, 1994.

                                            (ii) equals the sum of (I) plus (ID
                                         below.

                                                 (I) equals the accrued  benefit
                                            as of December 31, 1988, frozen in
                                            accordance with Treas. Reg. Section
                                            1.401(a)(4) -13.

                                                 (II) equals the accrued benefit
                                            determined under the Old Plan
                                            Benefit Formula applicable for the
                                            Plan Year beginning January 1, 1989,
                                            as applied to the Employee's years
                                            of Credited Service credited to the
                                            Employee for Plan


                                      -3-
<PAGE>   4
                                            Years beginning on or after January
                                            1, 1989 through December 31, 1993
                                            (to the extent recognized for
                                            computing a benefit under the Plan),
                                            frozen in accordance with Treas.
                                            Reg. Section 1.401(a)(4)-13.

                                        (B) equals the accrued benefit
                                     determined under the Old Plan Benefit
                                     Formula as applied to the Employee's years
                                     of Credited Service credited to the
                                     Employee for Plan years beginning on or
                                     after January 1, 1994 (to the extent
                                     recognized for computing a benefit under
                                     the Plan).

                  A Section 401(a)(17) Employee means an Employee whose current
         Accrued Benefit as of a date on or after January 1, 1989, is based on
         Compensation for a Plan Year beginning before January 1, 1994 that
         exceeded $150,000."

         Except as specifically amended hereby, the Plan shall remain in full
force and effect as prior to First Amendment.

         IN WITNESS WHEREOF, the Primary Sponsor has caused this First Amendment
to be executed as of the day and year first above written.


                                    BROWNING-FERRIS INDUSTRIES, INC.

                                    By:         /s/ Gerald K. Burger
                                          -----------------------------------
                                    Title:  Vice President and Secretary
                                          -----------------------------------

ATTEST:

By:       /s/ Eileen B. Schuler
       ----------------------------
Title:     Assistant Secretary
       ----------------------------



[CORPORATE SEAL]



                                      -4-

<PAGE>   5

                                                                   EXHIBIT 10.15


                               SECOND AMENDMENT TO
                               BFI RETIREMENT PLAN


         THIS SECOND AMENDMENT, made as of 31st day of December, 1995, by
BROWNING-FERRIS INDUSTRIES, INC., a Delaware corporation (the "Primary
Sponsor");


                                  INTRODUCTION

         The Primary Sponsor maintains the BFI Retirement Plan under an
indenture dated December 30, 1994, as amended (the "Plan"). The Primary Sponsor
desires to amend the Plan primarily to provide that a spouse of a deceased
participant may elect to receive immediately his or her distribution in the form
of a single life annuity.

         NOW THEREFORE, the Primary Sponsor does hereby amend the Plan as
follows:

         1. With respect to distributions in which the annuity starting date is
on or after the date of adoption of this Amendment, by substituting the
following for Plan Section 1.3(b):

                  "(b) for purposes of determining the present value Actuarial
                  Equivalent of the guaranteed minimum accrued benefit
                  determined pursuant to Section 1.2(a), the prevailing
                  commissioners' standard table (described in Code Section
                  807(d)(5)(A)) used to determine reserves for group annuity
                  contracts issued on the date as of which present value is
                  being determined (without regard to any other subparagraph of
                  Code Section 807(d)(5)) and the annual interest rate on
                  30-year Treasury securities, as specified by the Internal
                  Revenue Service, for the second full month preceding the Plan
                  Year in which the annuity starting date occurs shall be used."

         2. Effective January 1, 1996, by substituting the following for Plan
Section 6.3:
       
                  "6.3 Death Benefits of Married Participant Not in Pay Status.
                  If the Participant dies while married before the commencement
                  of retirement payments under the Plan, the Participant's
                  Spouse shall receive as a death benefit a Normal Fund Payment
                  payable commencing as of the earliest retirement age under the
                  Plan; provided, however that a Spouse whose Normal Fund
                  Payment is an annuity described in Plan Section 1.33(c) may
                  elect, prior to commencement of payments, to receive instead a
                  single life annuity payable in monthly installments for the
                  life of the Spouse or a lump sum, in either case payable
                  immediately, that is the Actuarial Equivalent."


<PAGE>   6


         Except as specifically amended hereby, the Plan shall remain in full
force and effect as prior to Second Amendment,

         IN WITNESS WHEREOF, the Primary Sponsor has caused this Second
Amendment to be executed as of the day and year first above written.


                       BROWNING-FERRIS INDUSTRIES, INC.

                       By:        /s/ Craig W. Wasserman
                              ----------------------------------------------
                       Title:    Vice President, Organizational Development
                              ----------------------------------------------

ATTEST:

By:       /s/ Eileen B. Schuler
        --------------------------
Title:     Assistant Secretary
        --------------------------


[CORPORATE SEAL]


                                      -2-






<PAGE>   7

                                                                   EXHIBIT 10.15


                     THIRD AMENDMENT TO BFI RETIREMENT PLAN


         THIS THIRD AMENDMENT, made on this 31st day of December, 1996, by
Browning-Ferris Industries, Inc., a Delaware corporation (the "Primary
Sponsor");


                                  INTRODUCTION

         The Primary Sponsor maintains the BFI Retirement Plan under an
indenture dated December 30, 1994 as amended by the First and Second Amendments
(the "Plan"). The Primary Sponsor now desires to amend the Plan to make changes
requested by the Internal Revenue Service and to provide that a Participant who
has reached Normal Retirement Age and who continues to be an Employee may elect
to begin receipt of his benefits.

         NOW, THEREFORE, the Primary Sponsor does hereby amend the Plan as
follows:

               1. Effective October 1, 1993, by substituting the following for
Section 1.1(b):

                  "(b) interest on the period's beginning balance computed by
                  multiplying such balance by the annual interest rate on
                  30-year Treasury securities for the third full month preceding
                  the beginning of that Plan Year (such annual interest rate for
                  July 1993 with respect to the beginning balance as of October
                  1, 1993), but not more than 6% per annum. (In the case of
                  Employees who are employed on the last day of the Plan Year,
                  such interest shall be credited as of the last day of each
                  Plan Year. In the case of Employees who have a Severance Date
                  during a Plan Year, such interest shall be credited as of the
                  last day of the month following the Severance Date, and as of
                  the last day of each Plan Year thereafter preceding the Plan
                  Year in which payment commences.) In either event, such
                  interest shall be credited for each Plan Year through the end
                  of the month coinciding with or immediately preceding the date
                  as of which payment commences under the Plan."

               2. By replacing the reference in Plan Section 1.3(b) to Section
1.2(a)" with "Section 1.2."

               3. By adding the following to Section 4.6, effective April 1, 
1996:

                  "4.6 Benefits While an Employee After Reaching Normal
                  Retirement Age. A Participant who continues as an Employee and
                  has reached Normal Retirement Age may elect to receive a
                  pension (determined as of the annuity starting date pursuant
                  to Plan Section 4.2 or 4.3, as applicable (but without regard
                  to the requirement that he retire), commencing as of the first
                  day of any month on or after his Normal 




<PAGE>   8


                  Retirement Age. Any benefit the Participant accrues after the
                  date as of which his Accrued Benefit is valued for payment for
                  these purposes shall be treated as a separate Accrued Benefit
                  which shall be paid in accordance with the provisions of the
                  Plan when the Participant actually terminates employment or
                  dies."

         4.       By adding the following new Section 7.8 effective April 1, 
1996:

                  "7.8 Participants Who Elect to Receive Benefits While Employed
                  After Reaching Normal Retirement Age. Subject to the
                  provisions of the Plan, a Participant who is entitled to
                  payment pursuant to Section 4.6 may elect any form of
                  distribution set forth in Plan Section 7.2(b); provided,
                  however, that once the Participant has begun to receive
                  payment in the elected form, such election shall remain
                  binding on the Participant with regard to his or his
                  Beneficiary's later entitlement to payment, after termination
                  of employment or death, of his pension attributable to his
                  separate Accrued Benefit which accrues after the date as of
                  which his pension is valued for the initial payment, and
                  payment of such additional separate benefit in such form shall
                  automatically be made as soon as feasible following
                  termination of employment or death."

         Except as specifically amended hereby, the Plan shall remain in full
force and effect as prior to this Third Amendment.

         IN WITNESS WHEREOF, the Primary Sponsor has caused this Third Amendment
to be executed as of the day and year first above written.


                                    BROWNING-FERRIS INDUSTRIES, INC.

                                    By:      /s/ Gerald K. Burger
                                          ----------------------------
                                    Title:   Vice President
                                          ----------------------------

ATTEST:

By:      /s/ Eileen B. Schuler
       --------------------------
Title:    Assistant Secretary
       --------------------------



[CORPORATE SEAL]


                                      -2-

<PAGE>   9
                                                                   EXHIBIT 10.15



                     FOURTH AMENDMENT TO BFI RETIREMENT PLAN

         THIS FOURTH AMENDMENT, made on this 31st day of December, 1996, by
Browning-Ferris Industries, Inc., a Delaware corporation (the "Primary
Sponsor");

                                  INTRODUCTION

         The Primary Sponsor maintains the BFI Retirement Plan under an
indenture dated December 30, 1994, as amended (the "Plan"). The Primary Sponsor
now desires to amend the Plan to clarify the exclusions from compensation.

         NOW, THEREFORE, the Primary Sponsor does hereby amend the plan as
follows:

         1.       By adding the following Subsection (i) to Section 1.11:

                  "(i) For purposes of determining a Participant's benefit
                  accrual in any Plan Year, income attributable to rodeo awards,
                  spot bonus awards, monetary gifts, Chairman's Club awards,
                  area wage premiums, allowances attributable to foreign
                  assignments and restricted stock grants shall be excluded,
                  unless determined to discriminatory under Code Section
                  401(a)(4)."

         2.       By adding the following Subsection (h) to Section 1.30 
         effective December 1, 1996:

                  "(h) Any Employee whose employment is transferred from
                  American-Ref-Fuel Company to a Plan Sponsor shall have his
                  hours of service with American Ref-Fuel Company counted as
                  Hours of Service for purposes of the definition of Eligibility
                  Service and Vesting Service."

         Except as specifically amended hereby, the Plan shall remain in full
force and effect as prior to this Fourth Amendment.

         IN WITNESS WHEREOF, the Primary Sponsor has caused this Fourth
Amendment to be executed as of the day and year first above written.


                                      BROWNING-FERRIS INDUSTRIES, INC.

                                      By:      /s/ Gerald K. Burger
                                              ----------------------------
                                      Title:   Vice President
                                              ----------------------------

ATTEST:

By:       /s/ Eileen B. Schuler
        ----------------------------
Title:     Assistant Secretary
        ----------------------------

[CORPORATE SEAL]


<PAGE>   10
                     FIFTH AMENDMENT TO BFI RETIREMENT PLAN



     The Fifth Amendment made on the 9th day of May, 1998, by Browing-Ferris
Industries, Inc., a Delaware corporation (the "Primary Sponsor");

                                  INTRODUCTION

     Primary Sponsor maintains the BFI Retirement Plan under an indenture dated
December 30, 1994, a amended (the "Plan").  The Primary Sponsor now desires to
amend the Plan to revise the formula under which benefits accrue.

     NOW THEREFORE, the Primary Sponsor does hereby amend the Plan, effective
January 1, 1998, as follows:

     1.  By replacing Section 1.1(a) with the following:

     "(a) 4.5% of Compensation paid from January 1, 1998 through May 31, 1998,
     2% of Compensation paid from June 1, 1998 through December 31, 1998, and
     thereafter, 2% of Compensation for the Plan Year, of a Participant who is 
     an Eligible Employee (and with respect to any temporary and seasonal 
     Employee only, has received a year of Credited Service for such Plan Year).
     (In the case of Employees who are employed as of the last day of the Plan 
     Year, such amount shall be credited as of the last day of each Plan Year.
     In the case of Employees who have a Severance Date during a Plan Year, such
     Compensation shall be credited as of the last day of the month following
     the Severance Date); plus"

     2.  By replacing Section 1.1(2) and (3) with the following:

     (2)  A Participant who reaches a Disability Retirement Date shall receive 
          a credit under Subsection (a) above equal to the credit provided 
          under the Plan prior to this Amendment for Plan Years ending before 
          January 1, 1998 through May 31, 1998, 7/12's of 2% from June 1, 1998
          through December 31, 1998, and thereafter 2%, of the greater of the 
          Participant's Compensation in the Plan Year in which the Disability 
          Retirement Date occurred or the first Plan year immediately preceding 
          that date, continuing for each Plan year until the earlier of the 
          date as of which the Participant (A) reaches age 65, (B) is no longer 
          subject to a Disability, or (C) elects to begin receiving payment 
          under the Plan; and

     (3)  A Participant who has not incurred a Severance Date and who is on an
          authorized medical leave of absence shall receive a credit under 
          Subsection (a) above for the Plan year in which the leave of absence
          started (A) in the case of the 1998 Plan Year, equal to 5/12's of
          4.5% from January 1, 1998 through May 31, 1998 and 7/12's of 2% from 
          June 1, 1998 through December 31, 1998, of the greater of the 
          Participant's Compensation in the Plan Year in which the leave of
          absence started or the first Plan Year immediately preceding that 
          date."

     Except as specifically amended hereby, the Plan shall remain in full force
and effect as prior to this Amendment.

     IN WITNESS WHEREOF, the Primary Sponsor has caused this Amendment to be
executed as of the day and year first above written.

                                          BROWNING-FERRIS INDUSTRIES, INC.

                                          By:     /s/ Rufus Wallingford
                                                 -----------------------------
                                              Rufus Wallingford
                                          
                                          Title:  Senior Vice President
                                                 -----------------------------

Attest:

By:    /s/ Eileen B. Schuler
       ------------------------------
       Eileen B. Schuler

Title: Assistant Secretary
       ------------------------------

        [CORPORATE SEAL]