<PAGE>   1
                                                                   EXHIBIT 10.21


                            EMPLOYMENT AGREEMENT

         This Employment Agreement (the "AGREEMENT") between ALLWASTE, INC., a
Delaware corporation (the "COMPANY"), and MICHAEL W. RAMIREZ (the "EMPLOYEE")
is hereby entered into effective as of the 11th day of November, 1996.

                                   RECITALS:

         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 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.

         NOW, THEREFORE, in consideration of the mutual covenants, promises and
agreements contained herein, and other good and valuable consideration, the
receipt and sufficiency of which is hereby acknowledged, the parties hereto
agree as follows:

1.       EMPLOYMENT AND RESPONSIBILITIES.

                 (a)      EMPLOYMENT.  The Company hereby employs Employee as
         its Vice President -- Controller.  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-
<PAGE>   2
        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.

                 (b)      AUTHORITY.  The Employee shall have authority
         commensurate with the authority normally accorded his position.  All
         actions of the Employee shall be in conformance with all policies of
         the Company in effect and consistent with the Company's policy manual.

                 (c)      AFFILIATES.  The Employee may from time to time be
         required to perform services for divisions, subsidiaries or affiliates
         of the Company, in which event the terms and conditions of this
         Agreement shall apply as if such affiliated company were a party to
         this Agreement.

                 (d)      SOLE EMPLOYMENT.  The Employee shall not, during the
         term of his employment hereunder, be engaged in any other business
         activity pursued for gain, profit or other pecuniary advantage if such
         activity interferes with Employee's duties and responsibilities
         hereunder.  This shall not, however, be construed as prohibiting the
         Employee from engaging in other activities and making personal
         investments which do not conflict with his responsibilities to the
         Company.

2.       COMPENSATION.

         For all services rendered by the Employee to the Company, the Company
shall compensate the Employee as follows:

                 (a)      BASE SALARY.  The base salary payable to the Employee
         under this Agreement shall be $110,000 per year, payable in equal bi-
         weekly installments or on any other periodic basis consistent with the
         Company's payroll procedures, which amount may be increased from time
         to time.

                 (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 incentive bonus plan, deferred compensation
                 plan and supplemental executive retirement plan, as each 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.





                                    Page 2 of 10
<PAGE>   3
                          (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.

3.       NONCOMPETITION AGREEMENT.


                 (a)      During the term of this Agreement and for a period of
         two (2) years immediately following the termination of this Agreement,
         the Employee shall not, for any reason whatsoever, directly or
         indirectly, for himself or on behalf of, or in conjunction with, any
         other person, persons, company, partnership, corporation or business
         of whatever nature:  (1) call on any customer of the Company, past or
         present, including, but not limited to, any customers obtained for the
         Company by the Employee, for the purpose of soliciting or selling any
         products or services in competition with those of the Company; (2)
         call on any employee of the Company for the purpose or with the intent
         of enticing them away from or out of the employ of the Company for any
         reason whatever; or (3) establish, enter into, be employed by or for,
         advise, consult with or become an owner in or a part of, any company,
         partnership, corporation or other business entity or venture, or in
         any way engage in business for himself or for others, in competition
         with the Company within 100 miles of the home office of the Company or
         its subsidiaries having a permanent and known facility wherein the
         Employee has served.

                 (b)      These covenants on the part of the Employee shall be
         construed as an agreement independent of any other provision of this
         Agreement, and the existence of any claim or cause of action of the
         Employee against the Company, whether predicated on this Agreement or
         otherwise, shall not preclude the Company's enforcement of this
         covenant.  In the event of a breach or threatened breach by the
         Employee of his obligations under this Section 3, the Employee
         acknowledges that the Company will not have an adequate remedy at law
         and shall be entitled to such equitable and injunctive relief as may
         be available to restrain the Employee from the violation of the
         provisions hereof.  Nothing herein shall be construed as prohibiting
         the Company from pursuing any other remedies available for such breach
         or threatened breach, including the recovery of damages from the
         Employee.





                                    Page 3 of 10
<PAGE>   4
                 (c)      It is agreed by the parties that the covenants set
         forth in this Section 3 impose a reasonable restraint on the Employee
         in light of the activities and business of the Company on the date of
         execution of this Agreement and in light of the future plans of the
         Company.  It is the intent of the parties that such covenants be
         construed and enforced in light of the activities and business of the
         Company on the date of termination of the Employee's employment with
         the Company.

                 (d)      The covenants in this Section 3 are severable and
         separate, and the unenforceability of any specific covenant shall not
         affect the provisions of any other covenant.  Moreover, in the event
         any court of competent jurisdiction shall determine that the scope,
         time or territorial restrictions set forth herein are unreasonable,
         then it is the intention of the parties that such restrictions be
         enforced to the fullest extent which the court deems reasonable and
         the Agreement shall thereby be reformed.

                 (e)      All of the covenants set forth in this Section 3
         shall be construed as an agreement independent of any other provision
         of this Agreement, and the existence of any claim or cause of action
         of the Employee against the Company, whether predicated on this
         Agreement or otherwise, shall not constitute a defense to the
         enforcement by the Company of such covenants.  It is agreed by the
         parties that the two-year period stated at the beginning of this
         Section 3, during which the agreements and covenants of the Employee
         contained herein shall be effective, shall be computed by excluding
         from such computation any time during which the Employee is in
         violation of any provision of this Section 3 and any time during which
         there is pending in any court of competent jurisdiction any action
         (including any appeal from any final judgment) brought by any person,
         whether or not a party to this Agreement, in which action, the Company
         seeks to enforce the agreements and covenants of the Employee or in
         which any person contests the validity of such agreement and covenants
         or their enforceability or seeks to avoid their performance or
         enforcement.

4.       RETURN OF COMPANY PROPERTY.

         All products, records, designs, patents, plans, manuals, "field
guides," memoranda, lists and other property delivered to the Employee by or on
behalf of the Company or by its customers (including, without limitation,
customers obtained for the Company by the Employee), together with all
correspondence with customers or representatives, reports, charts, records,
data and advertising materials compiled or collected by the Employee, which
pertain to the business of the Company (including, without limitation, all such
reports, charts, records, data and other materials contained in computer files,
disks, magnetic tape and other such media), shall be and remain the property of
the Company and be subject at all times to its discretion and control.  All
such property shall be delivered promptly to the Company without request on the
date the Employee is no longer employed by the Company.





                                    Page 4 of 10
<PAGE>   5
5.       INVENTIONS.

         The Employee shall disclose promptly to the Company any and all
conceptions and ideas for inventions, improvements and valuable discoveries,
whether patentable or not, which are conceived or made by the Employee solely
or jointly with others during the period of employment and which are related to
the business or activities of the Company or which the Employee conceives as a
result of his employment by the Company.  Further, the Employee hereby assigns
and agrees to assign all his interests therein to the Company or its nominee.
Whenever requested to do so by the Company, the Employee shall execute any and
all applications, assignments or other instruments which the Company shall deem
necessary to apply for and obtain letters patent of the United States or any
foreign country or to otherwise protect the Company's interest therein.  These
obligations shall continue beyond the end of employment with respect to
inventions, improvements and valuable discoveries, whether patentable or not,
conceived, made or acquired by the Employee during the period of employment and
shall be binding on the Employee's assigns, executors, administrators and other
legal representatives.

6.       TERM.

         The term of this Agreement shall begin on the date of this Agreement
and, unless terminated as herein provided, continue for a term of five (5)
years and thereafter on a year-to-year basis on the same terms and conditions
contained herein.

7.       TERMINATION; COMPENSATION AND OTHER RIGHTS ON TERMINATION.

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

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

                  (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.                                               
                                                                               




                                    Page 5 of 10
<PAGE>   6
                 (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 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 7(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 7(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





                                    Page 6 of 10
<PAGE>   7
                 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 7(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, 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 7(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.





                                  Page 7 of 10
<PAGE>   8
                 (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 7(e) shall be null and void.

                          (2)     Compensation and Benefits.  In the event the
                 Company elects to terminate this Agreement pursuant to this
                 Section 7(e), the Employee shall be entitled to receive his
                 base monthly salary for 12 months (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 Plan so that such options continue to vest in
                 accordance with the terms of the original grants during the
                 Severance Period 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) 12 months.  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 7 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 8
                 herein shall survive such termination, and except as otherwise
                 provided in this Section 7, the Employee shall thereafter have
                 no right to receive any compensation hereunder.

8.      TRADE SECRETS.

         The Employee agrees that he will not, during or after the term of his
employment with the Company, disclose the Company's distributors or customers
or any other trade secrets of the Company whether in existence or proposed, to
any person, firm, partnership, corporation or business for any reason or
purpose whatsoever.





                                    Page 8 of 10
<PAGE>   9
9.       COMPLETE AGREEMENT.

         This Agreement 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.  This written Agreement 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.

10.      NO WAIVER.

         No waiver by the parties hereto of any default or breach of any term,
condition or covenant of this Agreement shall be deemed to be a waiver of any
subsequent default or breach of the same or any other term, condition or
covenant contained herein.

11.      SEVERABILITY.

         The provisions of this Agreement shall be deemed severable and the
invalidity or unenforceability of any provision shall not affect the validity
and enforceability of the other provisions hereof.  If any provision of this
Agreement is unenforceable for any reason whatsoever, such provision shall be
appropriately limited and given effect to the extent that it may be
enforceable.

12.      GOVERNING LAW; PLACE OF PERFORMANCE.

         This Agreement shall in all respects be construed according to the
laws of the State of Texas.

13.      ATTORNEYS' FEES AND COSTS.

         If any action at law or in equity is brought to enforce or interpret
the terms of this Agreement, the prevailing party shall be entitled to
reasonable attorneys' fees, costs and necessary disbursements in addition to
any other relief to which it may be entitled.

14.      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 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





                                    Page 9 of 10
<PAGE>   10
entity and that any such entity shall automatically succeed to the rights,
powers and responsibilities of the Company hereunder.


         WITNESS, this Employment Agreement has been executed as of the date
first hereinabove written.

ALLWASTE, INC.                      EMPLOYEE


By: /s/ Robert M. Chiste            /s/ Michael W. Ramirez                    
    -----------------------         ------------------------------------------
    Robert M. Chiste                Michael W. Ramirez
    President and                   Social Security No.                       
    Chief Executive Officer                             ----------------------
                       
Date: November 11, 1996                                           
      ---------------------         Home Address:                              
                                                                               
                                    3146 Confederate South Drive               
                                    ------------------------------------------ 
                                    Missouri City, Texas  77459                
-----------------------             ------------------------------------------ 
Witness Signature                                                              
                                    Date: November 11, 1996
                                          ------------------------------------ 
                                                                               
                                                                               
                                    ------------------------------------------ 
                                    Witness Signature                          
                                                                               




                                 Page 10 of 10