4.4.4 Planning Effluent Management - Humanitarian Sanitation Hub

4.4.4 Planning Effluent Management

The chapter offers a rationale for managing the effluent and illustrates why post-treatment is required.

When determining how to manage the effluent, the global WASH cluster’s “Sanitation Quality Standards for Emergencies” offers guidance.

Rationale for Effluent Management (WASH Cluster, 2021)

Based on the decision tree provided, it has been concluded that the effluent can be infiltrated as all the required criteria have been met. However, to prevent the drain fields from clogging too quickly and to ensure that the recommended TSS level of less than 200mg/L is achieved, additional treatment steps such as Anaerobic Baffled Reactor (ABR) and Anaerobic Filter (AF) are implemented. 

Let’s determine if achieving the necessary TSS is possible by implementing all proposed treatment steps and if the resulting effluent would be safe for irrigating vegetable gardens.

The table below outlines the characteristics of the Influent (excreta and flush water) identified in the characterisation study.

Parameter INMedian ValueUnitSource
TSS7,900mg/LCharacterization study
COD21,200mg/LCharacterization study
TN2,200mg/LCharacterization study
E.coli500,000CFU/100mLCharacterization study
Design Assumptions for Influent

First Treatment Step: Biogas Digester


To predict the quality of the effluent, first, the typical treatment performance for the treatment unit needs to be understood – in our case, a fixed dome digester. The following performance values were collected from the literature review, as presented in the table below. Single values are used instead of a range to simplify the exercise objective.


ParameterValueUnitSource
TSS60%Literature
COD70%Literature
TN35%Literature
E.coli90%Literature
 Assumptions – Treatment Performance of the Fixed Dome Digester

We use a formula to predict the quality of the effluent by subtracting the reduced concentration from the influent concentration of each parameter, using the formula presented below.



Where,

ParameterDescriptionUnit
COUTConcentration of effluentmg/L or CFU/100mL
CINConcentration of influentmg/L or CFU/100mL
RRReduction rate%

Let’s apply the formula for predicting the TSS concentrations for the effluent.


When we apply the same formula for the remaining parameters, we get the following results.


ParameterValueUnitSource
TSS3,160mg/LCalculated
COD6,360mg/LCalculated
TN1,430mg/LCalculated
E.coli50,000CFU/100mLCalculated
Effluent Quality of the Fixed Dome Digester

The WHO standards for E.coli <1000 CFU/100mL  are not met for safe use.


Second Treatment Step: Anaerobic Baffled Reactor


Let’s calculate the anticipated quality of the effluent following ABR treatment based on the performance metrics outlined in the following table.

Is additional treatment with AF still required?


ParameterValueUnitSource
TSS70%Literature
COD80%Literature
TN15%Literature
E.coli80%Literature
Assumptions – Treatment Performance of the ABR

Applying the same calculation methodology as above, we get the following results.


ParameterRangeUnitSource
TSS950mg/LCalculated
COD1,275mg/LCalculated
TN1,215mg/LCalculated
E.coli10,000CFU/100mLCalculated
Effluent Quality of the ABR

The effluent quality does not meet the WHO standards and further treatment by AF is needed.


Third Treatment Step: Anaerobic Filter


Let’s calculate the anticipated quality of the effluent following AF treatment based on the performance metrics outlined in the table.

Is additional treatment required and is the effluent safe to use for irrigation?


ParameterRangeUnitSource
TSS60%Literature
COD80%Literature
TN20%Literature
E.coli80%Literature
Assumptions – Treatment Performance of the AF

Applying the same calculation methodology as for the previous treatment steps, we get the following results.

ParameterRangeUnitSourceWHO Effluent
Standards
TSS190mg/LCalculated<200
COD254mg/LCalculated<250
TN972mg/LCalculated<25
E.coli2,000CFU/100mLCalculated<1,000
Effluent Quality of the AF

The set treatment objective is only partially met (TSS), thus further treatment is required. The treated wastewater is not yet safe to use for the irrigation of low-risk crops.

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