EXHIBIT 4.01


                    AIR & WATER TECHNOLOGIES CORPORATION

                                    AND

                          THE CHASE MANHATTAN BANK

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

                       SECOND SUPPLEMENTAL INDENTURE


                       DATED AS OF FEBRUARY 23, 1998

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


                                     TO

                  THE INDENTURE DATED AS OF MAY 15, 1990,
                  AS AMENDED PURSUANT TO THE SUPPLEMENTAL
                      INDENTURE DATED OCTOBER 8, 1996,
              BETWEEN AIR & WATER TECHNOLOGIES CORPORATION AND
                THE CHASE MANHATTAN BANK (FORMERLY KNOWN AS
                   CHEMICAL BANK AND SUCCESSOR TRUSTEE TO
              MIDLANTIC NATIONAL BANK) AS TRUSTEE, RELATING TO
            $115 MILLION AGGREGATE PRINCIPAL AMOUNT AT MATURITY
             OF 8% CONVERTIBLE SUBORDINATED DEBENTURES DUE 2015




                           SUPPLEMENTAL INDENTURE

      THIS SECOND SUPPLEMENTAL INDENTURE (the "Supplemental Indenture") is
made as of the 23rd day of February, 1998, between Air & Water Technologies
Corporation (the "Company") and The Chase Manhattan Bank (formerly known as
Chemical Bank and successor Trustee to Midlantic National Bank), as trustee
(the "Trustee").

      WHEREAS, the Company and the Trustee heretofore executed and
delivered an Indenture, dated as of May 15, 1990 and amended pursuant to a
Supplemental Indenture dated October 8, 1996 (the "Indenture"); and

      WHEREAS, pursuant to the Indenture, the Company issued and the
Trustee authenticated and delivered $115 million aggregate principal amount
at maturity of the Company's 8% Convertible Subordinated Debentures due
2015 (the "Debentures"); and

      WHEREAS, Section 902 of the Indenture provides that with the consent
of the Holders of not less than a majority in principal amount of the
Outstanding Debentures at the time outstanding, the Company, when
authorized by Board Resolution, and the Trustee may enter into an indenture
or indentures supplemental thereto for the purpose of adding any provisions
to or changing in any manner or eliminating any of the provisions of the
Indenture or of modifying in any manner the rights of Holders subject to
certain exceptions identified therein; and

      WHEREAS, the Holders of not less than a majority in principal amount
of the Outstanding Debentures have duly consented to proposed modifications
set forth in this supplemental indenture (the "Supplemental Indenture") in
accordance with Section 902; and

      WHEREAS, the Company has heretofore delivered or is delivering
contemporaneously herewith to the Trustee (i) a copy of Board Resolutions
authorizing the execution, delivery and performance of this Supplemental
Indenture, (ii) evidence of the written consent of the Holders set forth in
the immediately preceding paragraph and (iii) an Opinion of Counsel in
compliance with and to the effect set forth in Sections 903 and 102 of the
Indenture; and

      WHEREAS, all conditions necessary to authorize the execution and
delivery of this Supplemental Indenture and to make this Supplemental
Indenture valid and binding have been complied with or have been done or
performed;

      NOW, THEREFORE, in consideration of the foregoing and notwithstanding
any provision of the Indenture which, absent this Supplemental Indenture,
might operate to limit such action, the Company and the Trustee agree as
follows for the equal and ratable benefit of the Holders of the Debentures:

                              ARTICLE I
                             DEFINITIONS

      SECTION 1.01. GENERAL. For all purposes of the Indenture and this
Supplemental Indenture, except as otherwise expressly provided or unless
the context otherwise requires:

      (a) the words "herein", "hereof" and "hereunder" and other words of
similar import refer to the Indenture and this Supplemental Indenture as a
whole and not to any particular Article, Section or subdivision; and

      (b) capitalized terms used but not defined herein shall have the
meanings assigned to them in the Indenture.

                                 ARTICLE II
                         AMENDMENT OF THE INDENTURE

      SECTION 2.01. AMENDMENT OF THE INDENTURE. Subject to Section 3.01
hereof, the Indenture is hereby amended in the following respects:

      (a) The definition of Change in Control in Section 1503(e) of the
Indenture is hereby amended by inserting "(other than Compagnie Generale
des Eaux and any of its Affiliates)" immediately following the word
"Person" in the second line of such paragraph 1503(e).

      (b) Section 1503(e) of the Indenture as amended pursuant to this
Supplemental Indenture shall read as follows:

         (e) a "Change in Control" of the Company shall be deemed to have
    occurred at such time as any Person (other than Compagnie Generale des
    Eaux and any of its Affiliates) is or becomes the beneficial owner,
    directly or indirectly, through a purchase, merger or other acquisition
    transaction or series of transactions, of shares of capital stock of
    the Company entitling such Person to exercise 75% or more of the total
    voting power of all shares of capital stock of the Company entitled to
    vote in elections of directors, provided that a Change in Control shall
    not be deemed to have occurred if either (i) the Quoted Price of Common
    Stock for any five trading days during the 10 trading day period
    immediately preceding the Change in Control shall equal or exceed 105%
    of the Conversion Price in effect on such day or (ii) all of the
    consideration (excluding cash payments for fractional shares) in the
    transaction or transactions constituting the Change in Control consists
    of shares of common stock traded on a national securities exchange or
    through NASDAQ or another comparable quotation system and as a result
    of such transaction or transactions the Debentures become convertible
    for such common stock.

                                ARTICLE III
                               MISCELLANEOUS

      SECTION 3.01. EFFECT OF SUPPLEMENTAL INDENTURE. Upon the execution
and delivery of this Supplemental Indenture by the Company and the Trustee,
the Indenture shall be supplemented in accordance herewith, and this
Supplemental Indenture shall form a part of the Indenture for all purposes,
and every Holder of Debentures heretofore or hereafter authenticated and
delivered under the Indenture shall be bound thereby.

      SECTION 3.02. INDENTURE REMAINS IN FULL FORCE AND EFFECT. Except as
supplemented hereby, all provisions in the Indenture shall remain in full
force and effect.

      SECTION 3.03. INDENTURE AND SUPPLEMENTAL INDENTURE CONSTRUED
TOGETHER. This Supplemental Indenture is an indenture supplemental to and
in implementation of the Indenture, and the Indenture and this Supplemental
Indenture shall henceforth be read and construed together.

      SECTION 3.04. CONFIRMATION AND PRESERVATION OF INDENTURE. The
Indenture as supplemented by this Supplemental Indenture is in all respects
confirmed and preserved.

      SECTION 3.05. CONFLICT WITH TRUST INDENTURE ACT. If any provision of
this Supplemental Indenture limits, qualifies or conflicts with any
provision of the Trust Indenture Act that is required under the Trust
Indenture Act to be part of and govern any provision of this Supplemental
Indenture, the provision of the Trust Indenture Act shall control. If any
provision of this Supplemental Indenture modifies or excludes any provision
of the Trust Indenture Act that may be so modified or excluded, the
provision of the Trust Indenture Act shall be deemed to apply to the
Indenture as so modified or to be excluded by this Supplemental Indenture,
as the case may be.

      SECTION 3.06. SEVERABILITY. In case any provision in this
Supplemental Indenture shall be invalid, illegal or unenforceable, the
validity, legality and enforceability of the remaining provisions shall not
in any way be affected or impaired thereby.

      SECTION 3.07. HEADINGS. The Article and Section headings of this
Supplemental Indenture have been inserted for convenience of reference
only, are not to be considered a part of this Supplemental Indenture and
shall in no way modify or restrict any of the terms or provisions hereof.

      SECTION 3.08. BENEFITS OF SUPPLEMENTAL INDENTURE, ETC. Nothing in
this Supplemental Indenture or the Debentures, express or implied, shall
give to any Person, other than the parties hereto and thereto and their
successors hereunder and thereunder and the Holders of the Debentures, any
benefit of any legal or equitable right, remedy or claim under the
Indenture, this Supplemental Indenture or the Debentures.

      SECTION 3.09. SUCCESSORS. All agreements of the Company in this
Supplemental Indenture shall bind its successors. All agreements of the
Trustee in this Supplemental Indenture shall bind its successors.

      SECTION 3.10. TRUSTEE NOT RESPONSIBLE FOR RECITALS. The recitals
contained herein shall be taken as the statements of the Company, and the
Trustee assumes no responsibility for their correctness. The Trustee shall
not be liable or responsible for the validity or sufficiency of this
Supplemental Indenture.

      SECTION 3.11. CERTAIN DUTIES AND RESPONSIBILITIES OF THE TRUSTEE. In
entering into this Supplemental Indenture, the Trustee shall be entitled to
the benefit of every provision of the Indenture relating to the conduct or
affecting the liability or affording protection to the Trustee, whether or
not elsewhere herein so provided.

      SECTION 3.12. GOVERNING LAW. The internal law of the State of New
York shall govern and be used to construe this Supplemental Indenture.

      SECTION 3.13. COUNTERPART ORIGINALS. The parties may sign any number
of copies of this Supplemental Indenture. Each signed copy shall be an
original, but all of them together represent the same agreement.

      IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed and attested, all as of the date and year
first above written.


                              AIR & WATER TECHNOLOGIES CORPORATION


                              By: /s/ Alain Brunais
                                 ---------------------------------
                              Name:   Alain Brunais
                              Title:  Chief Financial Officer


                              THE CHASE MANHATTAN BANK, AS TRUSTEE


                              By: /s/ R.J. Halleran
                                 ---------------------------------
                              Name:   R.J. Halleran
                              Title:  Second Vice President