OFFER TO PURCHASE FOR CASH
                 ANY AND ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF
                           TRIGEN ENERGY CORPORATION
                                       AT
                              $23.50 NET PER SHARE
                                       BY
                              T ACQUISITION CORP.
                     AN INDIRECT WHOLLY OWNED SUBSIDIARY OF
                                      ELYO
                     AN INDIRECT WHOLLY OWNED SUBSIDIARY OF
                            SUEZ LYONNAISE DES EAUX

  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
         TIME, ON FRIDAY, MARCH 24, 2000, UNLESS THE OFFER IS EXTENDED.

To Our Clients:

    Enclosed for your consideration are the Offer to Purchase, dated
February 28, 2000 (the "Offer to Purchase"), and the related Letter of
Transmittal (which, together with any amendments or supplements thereto,
collectively constitute the "Offer") relating to an offer by T Acquisition
Corp., a Delaware corporation ("Purchaser") and an indirect wholly owned
subsidiary of Elyo, a SOCIETE ANONYME organized and existing under the laws of
the Republic of France ("Parent") and an indirect wholly owned subsidiary of
Suez Lyonnaise des Eaux, a SOCIETE ANONYME organized and existing under the laws
of the Republic of France, to purchase any and all outstanding shares of Common
Stock, par value $0.01 per share (the "Shares"), of Trigen Energy Corporation, a
Delaware corporation (the "Company"), at a price of $23.50 per Share, net to the
seller in cash, without interest thereon, upon the terms and subject to the
conditions set forth in the Offer. The Offer is being made in connection with
the Agreement and Plan of Merger, dated as of January 19, 2000 (the "Merger
Agreement"), among Purchaser, Parent and the Company. This material is being
forwarded to you as the beneficial owner of Shares carried by us in your account
but not registered in your name.

    WE ARE (OR OUR NOMINEE IS) THE HOLDER OF RECORD OF SHARES HELD BY US FOR
YOUR ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF
RECORD AND PURSUANT TO YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED
TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER SHARES HELD
BY US FOR YOUR ACCOUNT.

    Accordingly, we request instructions as to whether you wish to have us
tender any or all of the Shares held by us for your account pursuant to the
terms and conditions set forth in the Offer.

    Please note the following:

    1. The tender price is $23.50 per Share, net to the seller in cash, without
interest.

    2. The Offer is being made for any and all of the outstanding Shares. There
is no minimum number of Shares required to be tendered.

    3. Tendering stockholders will not be obligated to pay brokerage fees or
commissions or, except as set forth in Instruction 6 of the Letter of
Transmittal, stock transfer taxes on the transfer of Shares pursuant to the
Offer. However, federal income tax backup withholding at a rate of 31% may be
required, unless an exemption is available or unless the required taxpayer
identification information is provided. See "Important Tax Information" of the
Letter of Transmittal.

    4. The special committee of the independent directors of the board of
directors of the Company (the "Company Board") has unanimously recommended that
the Company Board approve the Offer and the Merger (as defined in the Offer to
Purchase) and approve and adopt the Merger Agreement. The Company Board has
determined that the Merger is advisable and in the best interests of the
stockholders of the Company, and has approved the Merger. The Company Board has
also approved the Offer and recommended the Offer to the Company's stockholders.

    5. Notwithstanding any other provision of the Offer, payment for Shares
accepted for payment pursuant to the Offer will in all cases be made only after
timely receipt by Harris Trust Company of New York (the "Depositary") of
(a) certificates for Shares pursuant to the procedures set forth under "THE
TENDER OFFER--Procedures for Tendering Shares" of the Offer to Purchase or a
timely Book-Entry Confirmation (as defined in the Offer to Purchase) with
respect to such Shares, (b) a properly completed and duly executed Letter of
Transmittal (or a manually signed facsimile thereof) with any required signature
guarantees or an Agent's Message (as defined in the Offer to Purchase) in
connection with a book-entry delivery of Shares, and (c) any other documents
required by the Letter of Transmittal. Accordingly, tendering stockholders may
be paid at different times depending upon when certificates for Shares or
Book-Entry Confirmations are actually received by the Depositary.

    THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
TIME, ON FRIDAY, MARCH 24, 2000, UNLESS THE OFFER IS EXTENDED.

    If you wish to have us tender any or all of the Shares held by us for your
account, please so instruct us by completing, executing, detaching and returning
to us the instruction form set forth below. If you authorize the tender of your
Shares, all such Shares will be tendered unless otherwise indicated in such
instruction form. An envelope to return your instruction to us is enclosed.
PLEASE FORWARD YOUR INSTRUCTIONS TO US AS SOON AS POSSIBLE TO ALLOW US AMPLE
TIME TO TENDER YOUR SHARES ON YOUR BEHALF PRIOR TO THE EXPIRATION OF THE OFFER.

    The Offer is not being made to, nor will tenders be accepted from or on
behalf of, holders of Shares residing in any jurisdiction in which the making of
the Offer or acceptance thereof would not be in compliance with the securities
laws of such jurisdiction. However, Purchaser may, in its discretion, take such
action as it may deem necessary to make the Offer in any jurisdiction and extend
the Offer to holders of Shares in such jurisdiction.

    In any jurisdiction where the securities, blue sky or other laws require the
Offer to be made by a licensed broker or dealer, the Offer will be deemed to be
made on behalf of Purchaser by Lazard Freres & Co. LLC, the Dealer Manager for
the Offer, or one or more registered brokers or dealers licensed under the laws
of such jurisdiction.

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                          INSTRUCTIONS WITH RESPECT TO
                         THE OFFER TO PURCHASE FOR CASH
                 ANY AND ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF
                           TRIGEN ENERGY CORPORATION

    The undersigned acknowledge(s) receipt of your letter and the enclosed Offer
to Purchase, dated February 28, 2000, and the related Letter of Transmittal
(which, together with any amendments or supplements thereto, collectively
constitute the "Offer") in connection with the offer by T Acquisition Corp., a
Delaware corporation ("Purchaser") and an indirect wholly owned subsidiary of
Elyo, a SOCIETE ANONYME organized and existing under the laws of the Republic of
France ("Parent"), and an indirect wholly owned subsidiary of Suez Lyonnaise des
Eaux, a SOCIETE ANONYME organized and existing under the laws of the Republic of
France, to purchase any and all outstanding shares of Common Stock, par value
$0.01 per share (the "Shares"), of Trigen Energy Corporation, a Delaware
corporation (the "Company"), at a price of $23.50 per Share, net to the seller
in cash, without interest thereon, upon the terms and subject to the conditions
set forth in the Offer. Purchaser has been formed by Parent in connection with
the Offer and the transactions contemplated thereby. The Offer is being made in
connection with the Agreement and Plan of Merger dated as of January 19, 2000,
among Purchaser, Parent and the Company.

    This will instruct you to tender to Purchaser the number of Shares indicated
below (or if no number is indicated below, all Shares) which are held by you for
the account of the undersigned, upon the terms and subject to the conditions set
forth in the Offer.

Number of Shares to be Tendered:*
- ---------------------------------------------------------


                                            
                                               SIGN HERE

Account Number: ----------------------------
                                               --------------------------------------------

Date: ---------------------------------, 2000
                                               --------------------------------------------
                                                               Signature(s)

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

                                               --------------------------------------------
                                                              (Print Name(s))

                                               --------------------------------------------
                                                            (Print Address(es))

                                               --------------------------------------------
                                                    (Area Code and Telephone Number(s))

                                               --------------------------------------------
                                                        (Taxpayer Identification or
                                                        Social Security Number(s))


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*   Unless otherwise indicated, it will be assumed that all Shares held by us
    for your account are to be tendered.

                                       3