<PAGE>   1
                                                                   EXHIBIT 10.15

                    FIRST AMENDMENT TO EMPLOYMENT AGREEMENT

         This First Amendment to Employment Agreement (the "Amendment") is made
and entered into as of this 11th day of November, 1996, by and between
Allwaste, Inc., a Delaware corporation (the "Company"), and R. L. Nelson, Jr.
(the "Employee").

         WHEREAS, the Company and the Employee are parties to an Employment
Agreement dated October 23, 1986 (the "Agreement") which is attached hereto as
Addendum 1 and is incorporated herein in its entirety by reference, pursuant to
which the Employee has performed certain services to the Company; and

         WHEREAS, the Company and the Employee desire to amend the Agreement as
provided herein.

         NOW, THEREFORE, for and in consideration of the mutual covenants and
promises and representations contained herein, and other good and valuable
consideration, the receipt and sufficiency of which are acknowledged herein,
the Company and the Employee agree as follows:

1.       The following recitals are hereby added as a preface to the body of
         the Agreement:

                 The following statements are true and correct:

                          As of the date of this Agreement, the Company,
                 through its wholly-owned subsidiaries, is engaged in the
                 business of providing industrial and environmental services.

                          The Employee is or will be employed by the Company in
                 a confidential relationship pursuant to which the Employee, in
                 the course of his employment with the Company, will have
                 access to and will become aware of and familiar with certain
                 business, technical and other confidential information
                 pertaining to the Company's specific manner of doing business
                 and its future plans with respect thereto, including, without
                 limitation, information relating to pricing, customers,
                 suppliers, methods, techniques, processes, products, services
                 and know-how of the Company (collectively, the "CONFIDENTIAL
                 INFORMATION"), which Confidential Information has been or will
                 be established by and maintained at great expense to the
                 Company and is proprietary to and constitutes the trade
                 secrets and valuable goodwill of the Company.

                          The Employee recognizes that the Company's business
                 is dependent on such Confidential Information and that
                 disclosure of any of the Company's Confidential Information by
                 the Employee would have a detrimental effect on the Company's


                              Page 1 of 8 Pages
<PAGE>   2
                 business.  The protection of its Confidential Information is
                 of critical importance to the Company.

                          The Company will sustain great loss and damage if,
                 during the term of this Agreement and for a period of two (2)
                 years immediately following termination of this Agreement for
                 any reason, the Employee should violate any provision of
                 Section 3 of this Agreement.  The parties acknowledge that
                 monetary damages for any such loss would be extremely
                 difficult to measure.

2.       Subparagraph 1(a) of the Agreement is hereby amended by deleting the
subparagraph in its entirety and substituting the following in replacement
thereof:

                          (a)     EMPLOYMENT.  The Company hereby employs
                 Employee as its Chairman of the Board.  The Company's
                 President or Board of Directors may request that the Employee
                 serve in various capacities for the Company's subsidiaries;
                 however, in connection with such service, the Employee shall
                 not be requested to undertake duties and responsibilities that
                 are substantially different than those assigned to the
                 Employee as a result of his primary position with the Company
                 or that are unreasonable (or inconsistent with those given to
                 similarly-situated employees) considering the skills and
                 expertise of the Employee and the condition of the Company.
                 The Employee hereby accepts this employment under the terms
                 and provisions herein contained and agrees to devote his full
                 time, attention and efforts to promote and further the
                 business and services of the Company.  The Employee shall
                 faithfully adhere to, execute and fulfill all policies
                 (written and unwritten) established by the Company.

3.       Section 2 of the Agreement is hereby amended by deleting the
subparagraph in its entirety and substituting the following in replacement
thereof:

                          (a)     BASE SALARY.  The base salary payable to the
                 Employee under this Agreement shall be $250,000 during the
                 fiscal year beginning September 1, 1996 and ending August 31,
                 1997 and shall be $200,000 during the fiscal year beginning
                 September 1, 1997 and ending August 31, 1998, payable in equal
                 bi-weekly installments or on any other periodic basis
                 consistent with the Company's payroll procedures, which amount
                 may be increased or decreased from time to time at the
                 discretion of the Compensation Committee of the Company's
                 Board of Directors.





                               Page 2 of 8 Pages
<PAGE>   3
                          (b)     ADDITIONAL COMPENSATION.  The Employee is
                 eligible to receive additional compensation from the Company
                 as described below:

                                   (i)     The Employee shall be eligible to
                          participate in the Company's supplemental executive
                          retirement plan, as it may be in effect from time to
                          time.

                                  (ii)     Subject to the rules and regulations
                          applicable thereto and to the extent that his
                          position, tenure, salary, age, health and other
                          qualifications make him eligible to so participate,
                          the Employee shall be entitled to participate in the
                          Company's employee benefit programs.

                                  (iii)    The Employee shall be entitled to
                          receive stock option grants as and when authorized by
                          the Compensation Committee of the Company's Board of
                          Directors.

                                  (iv)     The Employee shall be entitled to
                          receive no less than three (3) weeks of vacation time
                          per year.

                                  (v)      The Employee shall be entitled to
                          receive such other executive perquisites from the
                          Company as are customary, including, without
                          limitation, club membership dues and personal
                          financial and tax planning and tax preparation
                          services, together with reimbursement for all
                          expenses reasonably incurred in the performance of
                          his duties, subject to submission of appropriate
                          documentation in accordance with the Company's
                          expense reimbursement policy in effect from time to
                          time.

4.       Section 6 of the Agreement is hereby amended by deleting the
subparagraph in its entirety and substituting the following in replacement
thereof:

                 6.       TERM; TERMINATION; COMPENSATION AND OTHER RIGHTS ON
                          TERMINATION.

                 The term of this Agreement shall begin on the date of this
         Agreement and, unless terminated as herein provided, continue until
         August 31, 1998.

                 (a)      TERMINATION AS A RESULT OF THE EMPLOYEE'S DEATH.

                                  (1)  This Agreement will terminate
                          automatically on the death of the Employee.





                               Page 3 of 8 Pages
<PAGE>   4
                                  (2)  Compensation and Benefits. The Company
                          shall pay to the Employee's beneficiary an amount
                          equal to accrued compensation owing to the Employee
                          on the date of his death (including, without
                          limitation, salary, pro rata bonus (if any and
                          subject to the terms and conditions of any applicable
                          bonus or incentive compensation plans), deferred
                          compensation and accrued vacation pay), together with
                          applicable death benefits, if any.  In accordance
                          with the Company's Amended and Restated 1989
                          Replacement Non-Qualified Stock Option Plan (as the
                          same may be amended from time to time, the "Option
                          Plan"), the Employee's beneficiary shall be entitled
                          to exercise all exercisable stock options held by the
                          Employee as of the date of death until the earlier of
                          (i) the one-year period following the date of death
                          or (ii) the date the option would otherwise expire.

                 (b)      TERMINATION BY THE COMPANY ON ACCOUNT OF DISABILITY.

                                  (1)  If, as a result of the Employee's
                          inability to perform his duties under this Agreement
                          (with or without reasonable accomodation) because of
                          illness, physical or mental disability, or other
                          incapacity which continues for an uninterrupted
                          period in excess of three (3) months or a cumulative
                          period of six (6) months in any twelve (12) month
                          period, and if, within thirty (30) days after the
                          Company has given the Employee written notice of the
                          Company's intention to terminate the Employee's
                          employment hereunder as a result of such incapacity,
                          the Employee shall not have returned to the full-time
                          performance of his duties hereunder, then the Company
                          may thereafter terminate the Employee's employment on
                          account of "DISABILITY"; provided, however, such
                          termination shall not by itself alter or impair the
                          Employee's rights as a "disabled employee" or
                          otherwise under any of the Company's employee benefit
                          plans.

                                  (2)  Compensation and Benefits.  The Company
                          shall pay to the Employee an amount equal to accrued
                          compensation owing to the Employee as of the date of
                          termination (including, without limitation, salary,
                          pro rata bonus (if any and subject to the terms and
                          conditions of any applicable bonus or incentive
                          compensation plans), deferred compensation and
                          accrued vacation pay). Subject to approval by the
                          Compensation Committee of the Company's Board of
                          Directors, the Company shall cause all stock options
                          held by the Employee to be regranted under the
                          Company's 1992 Limited Non-Qualified Stock Option
                          Plan (the "1992 Plan") so that such options continue
                          to vest and remain exercisable for a period of twelve
                          months following the date of





                               Page 4 of 8 Pages
<PAGE>   5
                          termination Whenever compensation is payable to the
                          Employee hereunder during a period in which he is
                          partially or totally disabled, and such Disability
                          would (except for the provisions hereof) entitle the
                          Employee to Disability income or salary continuation
                          payments from the Company according to the terms of
                          any plan or program presently maintained or hereafter
                          established by the Company, the Disability income or
                          salary continuation paid to the Employee pursuant to
                          any such plan or program shall be considered a
                          portion of the payment to be made to the Employee
                          pursuant to this Section 6(b)(2) and shall not be in
                          addition hereto.  If Disability income is payable
                          directly to the Employee by an insurance company
                          under the terms of an insurance policy paid for by
                          the Company, the amounts paid to the Employee by such
                          insurance company shall be considered a portion of
                          the payment to be made to the Employee pursuant to
                          this Section 6(b)(2) and shall not be in addition
                          hereto.

                  (c)     TERMINATION BY THE COMPANY FOR CAUSE.

                                  (1)  The Company may at any time during the
                          term of this Agreement, in its sole discretion,
                          terminate the Employee's employment with the Company
                          for "Cause."  For purposes of this Agreement, the
                          following shall constitute "CAUSE": (1) the Employee
                          willfully and continually fails to perform
                          substantially the Employee's duties with the Company
                          (other than any such failure resulting from the
                          Employee's incapacity due to physical or mental
                          illness), which failure continues unabated after a
                          written demand for substantial performance is
                          delivered to the Employee by the President or the
                          Chairman of the Board that specifically identifies
                          the manner in which the President or the Board
                          believes that the Employee has not substantially
                          performed the Employee's duties; (2) the Employee
                          willfully engages in gross misconduct that is
                          materially and demonstrably injurious to the Company;
                          or (3) the Employee is convicted of a felony crime by
                          a court of competent jurisdiction.

                                  For purposes of this Section 6(c), an act or
                          failure to act on the Employee's part shall be
                          considered "willful" if done or omitted to be done by
                          the Employee otherwise than in good faith and without
                          reasonable belief that the Employee's action or
                          omission was in the best interest of the Company.
                          Notwithstanding the foregoing, the Employee shall not
                          be deemed to have been terminated by the Company for
                          Cause unless and until the Company shall have
                          delivered to the Employee a copy of a resolution duly
                          adopted by the affirmative vote of not less than a
                          majority of the entire membership of the Board, at a
                          meeting of the Board called and held for the purpose
                          (after reasonable notice to the Employee and an
                          opportunity for the Employee,





                               Page 5 of 8 Pages
<PAGE>   6
                          together with the Employee's counsel, to be heard
                          before the Board), finding that, in the good faith
                          opinion of the Board, the Employee was guilty of
                          conduct set forth in clauses (a) or (b) of the second
                          sentence of this Section 6(c) and specifying the
                          particulars thereof in reasonable detail.

                                  (2)  Compensation and Benefits. The Company
                          shall pay to the Employee an amount equal to accrued
                          compensation owing to the Employee as of the date of
                          termination (including, salary and accrued vacation
                          pay).  In accordance with the Company's Option Plan,
                          the Employee shall be entitled to exercise all
                          exercisable stock options held by the Employee as of
                          the date of termination until the expiration of the
                          three-month period following such date of
                          termination.

                 (d)      TERMINATION BY THE EMPLOYEE.

                                  (1)  At any time after the execution of this
                          Agreement, the Employee may elect to terminate this
                          Agreement and the Employee's employment hereunder.


                                  (2)  Compensation and Benefits.  In the event
                          the Employee terminates this Agreement for any
                          reason, the Employee shall be entitled to receive an
                          amount equal to accrued compensation owing to the
                          Employee as of the date of termination (including,
                          without limitation, salary, pro rata bonus (if any
                          and subject to the terms and conditions of any
                          applicable bonus or incentive compensation plans),
                          deferred compensation and accrued vacation pay).  In
                          accordance with the Company's Option Plan, the
                          Employee shall be entitled to exercise all
                          exercisable stock options held by the Employee as of
                          the date of termination until the expiration of the
                          three-month period following such date of
                          termination.

                 (e)      TERMINATION BY THE COMPANY FOR OTHER THAN CAUSE.

                                  (1)      At any time after the execution of
                          this Agreement, the Company may, without Cause, elect
                          to terminate this Agreement and the Employee's
                          employment hereunder; provided, however, that in the
                          event that severance benefits are triggered by a
                          Change in Control under any Executive Severance
                          Agreement between the Company and the Employee, the
                          compensation and benefits otherwise payable to the
                          Employee under this Section 6(e) shall be null and
                          void.





                               Page 6 of 8 Pages
<PAGE>   7
                                  (2)      Compensation and Benefits.  In the
                          event the Company elects to terminate this Agreement
                          pursuant to this Section 6(e), the Employee shall be
                          entitled to receive his base monthly salary for the
                          period from the date of such termination through
                          August 31, 1998 (the "Severance Period"), payable in
                          accordance with the Company's customary payroll
                          procedures, as severance.  In addition, the Company
                          shall pay to the Employee an amount equal to accrued
                          compensation owing to the Employee as of the date of
                          termination (including, without limitation, salary,
                          pro rata bonus (if any and subject to the terms and
                          conditions of any applicable bonus or incentive
                          compensation plans), deferred compensation and
                          accrued vacation pay). Subject to the approval of the
                          Compensation Committee, the Company shall cause all
                          stock options held by the Employee to be regranted
                          under the Company's 1992 Limited Non-Qualified Stock
                          Option Plan (the "1992 Plan") so that such options
                          continue to vest and shall remain exercisable until
                          three months following the earlier of the date of
                          final vesting of any such option grant or the final
                          date of the Severance Period.  The Company shall also
                          pay to the Employee an amount equal to (a) the amount
                          of the monthly premium payment to continue coverage
                          for the Employee and the Employee's eligible
                          dependents under the Company's health insurance plan
                          under COBRA, multiplied by (b) the number of  months
                          of the Severance Period.  Further, the Employee shall
                          be credited with an additional 12 months of service
                          credit under the Company's Supplemental Executive
                          Retirement Plan (the "SERP").

                 (f)      SURVIVING OBLIGATIONS FOLLOWING TERMINATION.

                                  (1)      In the event of termination of this
                          Agreement for any reason provided in this Section 6
                          herein or if Employee resigns prior to the expiration
                          of the term of this Agreement, all rights and
                          obligations of the Company and the Employee under
                          this Agreement shall cease immediately, except that
                          Employee's obligations under Sections 3, 4, 5 and 7
                          herein shall survive such termination, and except as
                          otherwise provided in this Section 6, the Employee
                          shall thereafter have no right to receive any
                          compensation hereunder.

5.       The following Section 13 shall be added to the Agreement as follows:

         13.     ASSIGNMENT; BINDING EFFECT.

                          The Employee understands that he has been selected
                 for employment by the Company on the basis of his personal
                 qualifications, experience and skills.  The





                               Page 7 of 8 Pages
<PAGE>   8
                 Employee, therefore, agrees that he cannot assign his rights
                 and obligations hereunder or delegate his duties hereunder.
                 Subject to the preceding two sentences, this Agreement shall
                 be binding on and inure to the benefit of the parties hereto
                 and their respective heirs, successors and assigns.  It is
                 further understood and agreed that the Company may be merged
                 or consolidated with another entity and that any such entity
                 shall automatically succeed to the rights, powers and
                 responsibilities of the Company hereunder.

6.       This Amendment shall be governed by and construed in accordance with
the laws of the State of Texas.

7.       The Agreement, as amended by this Amendment, supersedes any and all
other agreements, either oral or in writing, between Company and the Employee
with respect to the employment of the Employee by the Company and contains all
of the representations, covenants and agreements between the Company and the
Employee with respect to such employment.  The Agreement, as amended hereby,
may not be later modified except by a further writing signed by the Company and
the Employee, and no term of this Agreement may be waived except by writing
signed by the party waiving the benefit of such term.

8.       Except as modified by this Amendment, all other terms of the Agreement
shall continue in full force and effect without modification.

         IN WITNESS WHEREOF, the parties have executed this First Amendment to
Employment Agreement in duplicate originals, effective as of November 11, 1996.

                                   ALLWASTE, INC.
                                                                              
                                                                              
                                   By:     /s/ Robert M. Chiste               
                                   --------------------------------------     
                                   Robert M. Chiste                           
                                   President and Chief Executive Officer      
                                                                              
                                                                              
                                   R. L. NELSON,  JR.                 
                                                                              
                                                                              
                                   /s/ R. L. Nelson, Jr.      
                                   ------------------------------     
                                   R. L. Nelson, Jr.                  
                                                                              
                                                                              
                                                                              
                                                                              

                               Page 8 of 8 Pages