<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-09-28</periodOfReport>

    <issuer>
        <issuerCik>0000884504</issuerCik>
        <issuerName>THERMOENERGY CORP</issuerName>
        <issuerTradingSymbol>TMEN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001403463</rptOwnerCik>
            <rptOwnerName>Quercus Trust</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1835 NEWPORT BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2>A109-PMB 467</rptOwnerStreet2>
            <rptOwnerCity>COSTA MESA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92627</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001391795</rptOwnerCik>
            <rptOwnerName>Gelbaum David</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1835 NEWPORT BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2>A109-PMB 467</rptOwnerStreet2>
            <rptOwnerCity>COSTA MESA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92627</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>Co-Trustee</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001403417</rptOwnerCik>
            <rptOwnerName>Gelbaum Monica Chavez</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1835 NEWPORT BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2>A109-PMB 467</rptOwnerStreet2>
            <rptOwnerCity>COSTA MESA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92627</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>Co-Trustee</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.24</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-09-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>2000000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2008-09-15</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>8333333</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8333333</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F3"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.24</value>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-02-11</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>250000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>250000.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-02-11</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1041667</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9375000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F4"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.24</value>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-06-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>150000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>150000.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-06-25</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>625000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10000000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F5"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.24</value>
                <footnoteId id="F5"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-09-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>280000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <footnoteId id="F7"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-09-28</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F5"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1166667</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11116667</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Warrants</value>
                <footnoteId id="F6"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.50</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-09-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1120000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F7"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-09-28</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-09-30</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1120000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1120000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F8"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>.75</value>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-09-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>2000000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2008-09-15</value>
            </exerciseDate>
            <expirationDate>
                <value>2013-09-30</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2666667</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2666667</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F9"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-02-11</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>250000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>250000.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-02-11</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F9"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F9"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F9"/>
                    <footnoteId id="F11"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Secured Convertible Promissory Note</value>
                <footnoteId id="F10"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F10"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-06-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>150000.00</value>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>150000.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-06-25</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F10"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F10"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F10"/>
                    <footnoteId id="F12"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On September 15, 2008, the Reporting Persons and Issuer entered into that certain Securities Purchase Agreement (the &quot;2008 SPA&quot;), pursuant to which the Issuer issued to the Reporting Persons a Secured Convertible Promissory Note in the principal amount of $2,000,000, with a conversion price of $0.75 per share and interest rate of ten percent (10%) per annum (the &quot;2008 Note&quot;) and a Warrant to purchase up to 4,000,000 Common Stock, with an exercise price of $1.25 (the &quot;2008 Warrant&quot;).  The total purchase price of securities issued pursuant to the 2008 SPA (including 2008 Note and 2008 Warrant) was $2,000,000.</footnote>
        <footnote id="F2">On September 28, 2009, the Issuer issued to the Reporting Persons two 8% Secured Convertible Promissory Notes in the aggregate principal amount of $280,000 (the &quot;September 2009 Notes&quot;) and, in connection therewith, the 2008 Note was amended and restated with the following principal terms:  the conversion price was reduced to $0.24 per share, the interest rate was reduced to eight percent (8%) per annum and the maturity date was amended to mature on the earlier to occur of (i) the closing of the Second Tranche of the Series B Convertible Preferred Stock financing contemplated by that certain Term Sheet dated September 16, 2009 and executed by the Reporting Persons and certain other investors or (ii) December 31, 2010 (hereinafter, the &quot;New Maturity Date&quot;).</footnote>
        <footnote id="F3">On February 11, 2009, the Issuer issued to the Reporting Persons a Secured Convertible Promissory Note in the principal amount of $250,000 (the &quot;February 2009 Note&quot;).  The original terms and conditions of the February 2009 Note are more particularly set forth in the Issuer's Form 8-K filed on 2/17/2009, which also contains a copy of the February 2009 Note as an exhibit thereto.  In connection with the issuance of the September 2009 Notes, the February 2009 Note was amended and restated with the following principal terms:  the conversion price was amended to $0.24 per share, the interest rate was reduced to eight percent (8%) per annum and the maturity date was amended to the New Maturity Date.</footnote>
        <footnote id="F4">On June 25, 2009, the Issuer issued to the Reporting Persons a Secured Convertible Promissory Note in the principal amount of $150,000 (the &quot;June 2009 Note&quot;).  The original terms and conditions of the June 2009 Note are more particularly set forth in the Issuer's Form 8-K filed on 6/30/2009, which also contains a copy of the June 2009 Note as an exhibit thereto.  In connection with the issuance of the September 2009 Notes, the June 2009 Note was amended and restated with the following principal terms:  the conversion price was amended to $0.24 per share, the interest rate was reduced to eight percent (8%) per annum and the maturity date was amended to the New Maturity Date.</footnote>
        <footnote id="F5">The September 2009 Notes, which consists of one Secured Convertible Promissory Note in the principal amount of $250,000 and another in the principal amount of $30,000, have the following principal terms:  a conversion price of $0.24 per share, the interest rate of eight percent (8%) per annum and the maturity date of the New Maturity Date.  Additional terms and conditions of the September 2009 Notes are more particularly set forth in the Issuer's Form 8-K filed on 10/2/2009, which also contains a copy of a form of the September 2009 Notes as an exhibit thereto.</footnote>
        <footnote id="F6">On September 28, 2009, in connection with the issuance of the September 2009 Notes, the Reporting Persons acquired from the Issuer two Warrants, one to purchase up to 1,000,000 Common Stock and another to purchase up to 120,000 Common Stock (the &quot;September 2009 Warrants&quot;).  Additional terms and conditions of the September 2009 Warrants are more particularly set forth in the Issuer's Form 8-K filed on 10/2/2009, which also contains a copy of a form of the Warrants as an exhibit thereto.</footnote>
        <footnote id="F7">The total purchase price of the September 2009 Notes and the September 2009 Warrants was $280,000.</footnote>
        <footnote id="F8">As explained in Comment #1, the 2008 Note was amended and restated in connection with the September 2009 Notes.  Therefore, we hereby report the disposition of the 2,666,667 Common Stock of the Reporting Persons, which was based on the original conversion price of $.75.</footnote>
        <footnote id="F9">As explained in Comment #2, the February 2009 Note was amended and restated in connection with the September 2009 Notes.  Therefore, we hereby report the disposition of the February 2009 Note with its original conversion and maturation terms which are described in the Issuer's Form 8-K filed on 2/17/2009.</footnote>
        <footnote id="F10">As explained in Comment #3, the June 2009 Note was amended and restated in connection with the September 2009 Notes.  Therefore, we hereby report the disposition of the June 2009 Note with its original conversion and maturation terms which are described in the Issuer's Form 8-K filed on 6/30/2009.</footnote>
        <footnote id="F11">The February 2009 Note was not convertible into Common Stock at the time of its issuance.</footnote>
        <footnote id="F12">The June 2009 Note was not convertible into Common Stock at the time of its issuance.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ David Gelbaum, Co-Trustee, The Quercus Trust</signatureName>
        <signatureDate>2009-10-09</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ David Gelbaum, as attorney-in-fact for Monica Chavez Gelbaum, Co-Trustee, The Quercus Trust</signatureName>
        <signatureDate>2009-10-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
