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  <xsd:element name="SignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_SignificantAccountingPoliciesLineItems" substitutionGroup="xbrli:item" />
  <xsd:element name="ExpirationPeriodAxis" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ExpirationPeriodAxis" substitutionGroup="xbrldt:dimensionItem" />
  <xsd:element name="ExpirationPeriodDomain" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ExpirationPeriodDomain" substitutionGroup="xbrli:item" />
  <xsd:element name="CylindersMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_CylindersMember" substitutionGroup="xbrli:item" />
  <xsd:element name="BalanceSheetClassificationAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_BalanceSheetClassificationAbstract" substitutionGroup="xbrli:item" />
  <xsd:element name="OtherInformationAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_OtherInformationAbstract" substitutionGroup="xbrli:item" />
  <xsd:element name="StockOptionPlanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_StockOptionPlanMember" substitutionGroup="xbrli:item" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAbstract" substitutionGroup="xbrli:item" />
  <xsd:element name="TwoThousandAndFourteenStockIncentivePlanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_TwoThousandAndFourteenStockIncentivePlanMember" substitutionGroup="xbrli:item" />
  <xsd:element name="TwoThousandAndEighteenStockIncentivePlanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_TwoThousandAndEighteenStockIncentivePlanMember" substitutionGroup="xbrli:item" />
  <xsd:element name="FCCRMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_FCCRMember" substitutionGroup="xbrli:item" />
  <xsd:element name="SwingLineLoanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_SwingLineLoanMember" substitutionGroup="xbrli:item" />
  <xsd:element id="hdsn_FixedChargesCoverageRatio" name="FixedChargesCoverageRatio" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="hdsn_OneCustomerMember" name="OneCustomerMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_NoCustomerMember" name="NoCustomerMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_AmendedAndRestatedRevolvingCreditAndSecurityAgreementMember" name="AmendedAndRestatedRevolvingCreditAndSecurityAgreementMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_DebtInstrumentAdditionalPercentagePointsInterest" name="DebtInstrumentAdditionalPercentagePointsInterest" nillable="true" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" />
  <xsd:element id="hdsn_TermLoanFacilityMember" name="TermLoanFacilityMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_PropertyLocationAxis" name="PropertyLocationAxis" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_PropertyLocationDomain" name="PropertyLocationDomain" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_MinimumAggregateUndrawnLoanAvailability" name="MinimumAggregateUndrawnLoanAvailability" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" />
  <xsd:element id="hdsn_IncreaseDecreaseInLeaseObligations" name="IncreaseDecreaseInLeaseObligations" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="debit" />
  <xsd:element id="hdsn_WellsFargoMember" name="WellsFargoMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_LineOfCreditFacilityCovenantComplianceMinimumLiquidity" name="LineOfCreditFacilityCovenantComplianceMinimumLiquidity" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="hdsn_LineOfCreditFacilityCovenantComplianceMinimumLiquidityToBeDerivedFromAvailability" name="LineOfCreditFacilityCovenantComplianceMinimumLiquidityToBeDerivedFromAvailability" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="hdsn_PeriodForFixedChargeCoverageRatio" name="PeriodForFixedChargeCoverageRatio" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="hdsn_CylinderDepositValue" name="CylinderDepositValue" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" abstract="false" />
  <xsd:element id="hdsn_TwoThousandAndTwentyStockIncentivePlanMember" name="TwoThousandAndTwentyStockIncentivePlanMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonVested" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonVested" nillable="true" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="AccruedExpensesAndOtherCurrentLiabilitiesAbstract" id="hdsn_AccruedExpensesAndOtherCurrentLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SecuredOvernightFinancingRateMember" id="hdsn_SecuredOvernightFinancingRateMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="RevolvingCreditFacilityAmendmentMember" id="hdsn_RevolvingCreditFacilityAmendmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="FirstInLastOutTrancheMember" id="hdsn_FirstInLastOutTrancheMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SecuredOvernightFinancingRateAndFirstInLastOutTrancheMember" id="hdsn_SecuredOvernightFinancingRateAndFirstInLastOutTrancheMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PrimeCommercialLendingRateOfWellsFargoMember" id="hdsn_PrimeCommercialLendingRateOfWellsFargoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PercentageOfStepdownFromBaselineLevelsFinalRule" id="hdsn_PercentageOfStepdownFromBaselineLevelsFinalRule" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="PercentageOfCumulativeReductionFromBaselineLevelsSubsequentAllowance" id="hdsn_PercentageOfCumulativeReductionFromBaselineLevelsSubsequentAllowance" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="AllocationAllowanceReceivedByCompanyAsPercentageOfHydrofluorocarbonsConsumption" id="hdsn_AllocationAllowanceReceivedByCompanyAsPercentageOfHydrofluorocarbonsConsumption" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="AdjustmentsToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet" id="hdsn_AdjustmentsToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="ExcessTaxBenefitsFromShareBasedCompensationFinancingActivities" id="hdsn_ExcessTaxBenefitsFromShareBasedCompensationFinancingActivities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="NatureOfOperationsPolicyPolicyTextBlock" id="hdsn_NatureOfOperationsPolicyPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="CylinderDepositLiabilityPolicyPolicyTextBlock" id="hdsn_CylinderDepositLiabilityPolicyPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ShortTermAndLongTermDebtTable" id="hdsn_ShortTermAndLongTermDebtTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ShortTermAndLongTermDebtLineItems" id="hdsn_ShortTermAndLongTermDebtLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonvested." id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonvested." type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="LeaseObligationsCurrent" id="hdsn_LeaseObligationsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ScheduleOfSupplementalCashFlowAndNonCashInformationRelatedToLeasesTableTextBlock" id="hdsn_ScheduleOfSupplementalCashFlowAndNonCashInformationRelatedToLeasesTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="AccruedLiabilitiesAndOtherLiabilitiesCurrent" id="hdsn_AccruedLiabilitiesAndOtherLiabilitiesCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
</xsd:schema>
