4. RECOMMEN­DATIONS

4.1 European and National Regulations

4.1.1 Existing Measures at European Level and Reported Weaknesses

• The presence of plastic litter in the environment is increasingly considered as an indicator of good environmental status, such as descriptor 10 of the EU Marine Strategy Framework Directive 2008/56/EC (MSFD) "Marine litter - Properties and quantities of marine litter do not cause harm to the coastal and marine environment". This type of measure involves the establishment of monitoring protocols to quantify and identify reduction targets.
•  The Water Framework Directive 2000/60/EC (WFD) does not include plastic waste as an indicator of good ecological status (GES) in rivers. However, it is proven that rivers are the main transfer pathway from land-based sources to the ocean. As a result, there is a visible lack of coordination between the WFD and the MSFD and consequently a lack of preventive measures at the catchment basin scale.
•  The current Urban Wastewater Directive 91/271/EEC (UWWD), which regulates the discharge of treated wastewater into the environment, does not include targets for the reduction of plastic/microplastic waste in discharged water and does not take into account the presence of plastic biocarriers in the installation operations.

4.1.2 Recommendations – Implementation of New Measures

Adoption of ambitious measures

Integration of plastic waste as an indicator of the good ecological status of the waters targeted by the WFD.

4.2 Administrative Procedure for Discharge Authorisation

4.2.1 Existing Measures and Reported Weaknesses

• At present, only general details (effluent to be treated, general process, treatment capacity) of the biological treatment processes used are provided to the authorities responsible for authorising discharges into the environment.
• The authorities responsible for the approval of wastewater treatment plant applications are not always sufficiently trained to analyse the technical specifications of the wastewater treatment systems to be installed.
• Information on the characteristics of treatment plants and details of biological processes (e.g. type and volume of biocarriers used) are not collected in a central database and data on industrial and individual systems are not readily available.

4.2.2 Recommendations – Evolution of discharge authorisation procedure

Training the authorities in charge of the discharge permit

• Inform relevant authorities of the risks associated with biocarriers so that they can make an informed assessment of the application for the WWTP at an early stage. They could then guarantee that risk prevention measures are adopted.

Additional requirements for the application

• Report on the technologies used for biological wastewater treatment by WWTP manufacturers and/or operators, including, if applicable, the type and volume of plastic biocarriers.  

• Description of the specific retention devices deployed to prevent biocarrier leakage both within the plant and into aquatic environments and the integration of control measures into the operator's self-monitoring.

Request a hazard identification and risk assessment for each WWTP

• The approval of a discharge permit by the competent authorities should be conditional on a Hazard Identification and Risk Analysis (HIRA) for a particular system.

• Instructing bodies will need to ensure that two key points are incorporated into the HIRA:
  • Biocarriers are considered a potential environmental hazard.
  • The identification of preventive measures to minimise the risks and be better prepared in case of an incident.

4.3 Production, Transport, Storage and Handling

4.3.1 Reported Weaknesses

4.3.2 Recommendations - Safe Transport and Storage

Production and storage site design

• Equip production and storage sites with smooth floors to enable easier cleaning of surfaces in case of incidents. Loose, permeable flooring such as gravel should be avoided.

• Install removable collection screens and baskets over all surface water drainage outlets to stop and collect biocarriers in the event of spillage. These protections should be easy to install and easily accessible as soon as a spill occurs.

Storage conditions

• Store biocarriers in high-strength containers without risk of damage or breakage during handling.
• Big bags (1m³) as well as smaller bags (100L) could be considered.
• Prefer bags with eyelets and sleeves to allow handling by cranes and cables for better stability. This type of bags should allow for better control during the loading of the biocarriers into the tanks.
• Avoid packaging sensitive to sunlight or water. UV rays may strongly deteriorate materials (especially plastics), so storage of biocarriers in a sheltered area is recommended. If stored outside, the bags should be covered with properly secured additional tarpaulin.
• Ensure the use of suitable, sealed containers during each stage of biocarrier transport and storage, and check container integrity at each stage.
• Keep biocarriers out of areas susceptible to natural hazards. Prefer closed buildings to outdoor sites. If storage in flood-prone areas is necessary, it should not exceed 2 weeks and special monitoring should be carried out with daily checks of the weather forecast and flood risk.
• Plan the delivery of the biocarriers to the WWTP just before introduction into the tanks to avoid long storage times at the treatment plant. Storage time on the ground should be as short as possible.

Handling

• At the site of production load biocarriers into bags with a pneumatic device to improve filling accuracy and reduce the risk of overflow.

• Minimise the transport and handling of biocarriers throughout the supply chain.

Loading of biocarriers into the tanks

• When loading biocarriers into tanks, some commonsense parameters should be considered:
  • prefer a windless day;
  • use stable and appropriate handling equipment;
  • do not transfer the biocarriers to an already-full tank.

Training of on-site managers and staff

• Train staff in good bagging, handling, storage, containment, and spill recovery practices.
• Train staff in crisis management.

Emergency plan within the production site

• About the risk of spillage within the production or storage site :
  • Add the risk of biocarrier spillage to existing contingency plans.
  • Equip production and storage sites with containment and cleaning equipment such as industrial vacuum cleaners and spare recovery tanks to allow facility managers to quickly and easily recover from container damage or other minor spills within their working area.
  • Establish a list of priority measures in the event of biocarrier-related incident to contain the spill to the production or storage site.
  • Report any spills in the production/storage area to the company's safety officers.
• In case of spillage into the natural environment
  • Map nearby rivers and water bodies and their catchments to identify dams and areas where temporary dams can be placed (refer to existing maps and procedures for the diffusion of oil pollution, floating waste, etc.).
  • Report any losses of biocarriers as soon as possible in order to implement appropriate containment and clean-up measures at the production or storage site and in the surrounding environment.
  • Report any loss of biocarriers in the environment outside the company premises to the local environmental authorities.

Control

• Demonstrate facility compliance. All companies in the supply chain should regularly report and be audited on the prevention, containment, and clean-up procedures in place within their facilities.

4.4 Securing Installations - Performing the Hazard Identification & Risk Analysis (HIRA)

4.4.1 Definition of the Objectives of the HIRA

• cataloguing of sensitive equipment and interventions that may lead to malfunction.
• analysis of the impact of maintenance periods and major repairs
• proposals for corrective action adapted to each scenario:
  • in terms of functional architecture,
  • the particular specifications of the equipment,
  • detection and warning systems
• the list of parts to be made available or the availability of spare parts off-site,
• organisation and timing of the maintenance policy.

4.4.2 Responsibility for Risk Analysis

4.4.3 Methodology and Content

What are the principal failures to be considered?

• the frequency of the failure
• the severity of the failure (or its consequences)
• the detectability of the failure can also be included in the scoring.

• Any spillage, however small, at a high frequency (to be defined in consultation with the national or local water agency) should result in an unacceptable rating. The definition of the frequency will be based on the operator's experience and on the analysis of all available information (operating logs, accident reports, monitoring records, etc.).
• The combination of the severity and frequency factors enables fine gradation of the risk over a broad scale and the differentiation of incidents according to their risk level.
• Whatever rating system is chosen for a WWTP, it is important to maintain the same model over time so that changes can be prioritised and monitored temporally.

What should the HIRA deliverable contain?

• A description of the methodology used for the study (scoring method, scoring grid, etc.)
• The scope of the study (description of the system/network) and any justified exclusions
• A list of people involved in the study
• A summary of the risks identified (with a hierarchy according to the rating), particularly highlighting weak points: the most critical equipment or installations
• A summary of the recommendations at the end of the study (with a hierarchy in relation to the presented risks) relating to the technical, human, or organisational devices and provisions contributing to the control of the malfunction risk, concerning:
  • Instrumentation and automation: addition of sensors, safety level gauges, low/high threshold alarms, and all the instrumentation to allow good remote monitoring of the system.
  • Equipment: changes of or modification of critical equipment such as doubling pumps, overflow installations, and storage of emergency equipment or spare parts.
  • Operating instructions defining procedures for each type of malfunction. These may include the designation of the emergency power supply, instructions for the exchange of damaged equipment, diversion of flows, who to notify in the event of a crisis, etc.

• Weighting of the recommendation on risk reduction. Some recommendations are likely to reduce the frequency/probability of accidental scenarios or their consequences. Therefore, the recommendations that reduce the most critical risks should be prioritised.
• Effectiveness of prevention and protection measures. It is common practice to rank the effectiveness of a measure in the following hierarchy, from most to least effective:
  • Eliminate the hazard (stop use of a product or equipment...)
  • Replace a hazard with a lesser hazard.
  • Reduce risks at the source by modifying the design of a process or the operating parameters to make it more suitable for maintenance, for example.
• Organisational and management measures (awareness raising, training, changes in working and supervision methods, planning and organisation of tasks, signposting, etc.).
• Expected cost/benefit ratio: risk analysis is not intended to introduce disproportionately costly measures, but rather to ensure that the main deficiencies are met with the appropriate responses.

 4.4.4 Use of the HIRA

From the perspective of permit-issuing authorities

• Make the HIRA an official document to be sent to permit-issuing authorities to allow for tighter control of risks from the early stages of the project. Review by competent authorities would also allow, depending on the findings, to request additional supplies, equipment, or software appropriate and proportionate to the identified risks. 

From the perspective of the WWTP designer or operator

• For a new installation, the HIRA would allow certain design choices to be approved or modified.
• For existing installations, risk analysis would lead to an action plan to improve the current situation.

4.5 Design and Establishment

4.5.1 Existing Measures and Reported Weaknesses – Related to WWTP Engineering

• Collection and storage of effluent,
• Primary treatment (screening, grit removal, degreasing) 
• Secondary treatment (biological treatment, MBBR)
• Tertiary treatment (physicochemical)
• Quaternary treatment (removal of other chemical molecules and faecal bacteria)

• maintaining appropriate physicochemical conditions for the proper development of activated sludge,
• ensuring the reliability of the installations, processes, and equipment required to keep the biocarriers in the tanks.

• Wasterwater input;
• Mixing of effluents;
• Aeration of the biological treatment;
• Sensors (O2, water level, etc.);
• Retention measures (missing sieves or meshes);
• The structure of the tanks (type of materials);
• Pumps (sizing/backup pumps, etc.).

4.5.2 General Recommendations

• Strict quality requirements for civil engineering
• Correct sizing of ventilation and mixers
• Global flow management, including collection networks 
• Protection of outgoing flows
• Easy access for maintenance allowing the control of biocarriers at any time
• Appropriate training for good knowledge transfer from designer to operator.

4.5.3 Recommendations - Rainwater Management

Separating the collection of rainwater and domestic wastewater

• Prefer separate networks to maintain optimal treatment of domestic wastewater even in the case of heavy rainfall.

Development of stormwater management plans

• Increase the permeability of cities by creating larger vegetated areas, ditches, or green roofs to naturally direct water into the soil.
• Create natural open stormwater basins or underground buffer tanks in the collection network.

4.5.4 Recommendations - Location of the WWTP and its External Environment

Natural disasters protection

• Install lighting rods to prevent damage to electrical and control systems
• Avoid building in flood-prone areas. If this is not possible, some equipment such as electrical and control centres should be elevated. Provisions should also be made for pumping and/or evacuating water in the event of flooding.
• Carry out geotechnical studies to ensure the building durability and confirm the bearing capacity of the soil in support of the structures.

Malicious acts

• Installing detection and intruder alarm systems.
• Ensure cyber security of the WWTP's control or management tools.

Surrounding natural features

• Avoid installing open basin near deciduous trees to prevent leaves from interfering with biological functioning, clogging pipes, and pumping systems.

4.5.5 Recommendations - Structural Engineering

• Prefer closed systems for new construction.
• Anticipate the installation of easy-access hatches to facilitate maintenance of ventilation equipment and grids inside the tank.

  Castanie, S., 2016, Mise en service de la station d’épuration d’Ota Porto, Master's thesis at the Ecole Nationale du Génie de l’Eau et de l’Environnement of Strasbourg.
• Build the inner linings of the tanks to be as smooth as possible. This prevents organic matter build-up and the advent of stagnation zones in the effluent favourable for the development of filamentous bacteria.
• Install a smooth floor around the pools for quick, mechanical cleaning of the surfaces in case of leaks. Flooring material should provide some traction and gravel should be avoided.
• Raise the height of the walls of basins (open tanks) containing biocarriers.
• Equip the perimeter of the tanks with fine-mesh screens to catch biocarriers in the event of overflow or foam formation.

4.5.6 Recommendations - Flow Management

Flow velocity

• Maintain sufficient flow in the collection system.  Installation of treatment stations (addition of oxidants, metallic salts) in the collectors can be considered to avoid sulphide formation.

Buffer tank

• Provide a buffer tank for dilution, aeration, or the addition of reagents. This tank should be entirely emptiable and equipped with a pumping pit.

Design of lift stations and network lift stations

• Place the pump at the lowest point to avoid any organic matter accumulation;
• Provide sufficient slope at the bottom of the tank to carry the deposits to the pumping point;
• Mix sufficiently to prevent deposits;
• Ensure that there is no roughness on the tank walls to limit undesired bacterial growth.

Stormwater basin - Stormwater storage tank

• Install a storm water basin upstream of the plant to regulate flow.

Storm overflow

• Equip the plant with a bypass system to maintain constant flow in the event of large inflows and regulate flow without endangering the treatment process. Manual/mechanical systems should be activatable in the event of a power failure and guarantee the possibility of discharge if necessary.

Reactors in series or in parallel

• For larger installations, series reactors and/or several parallel lines should be preferred to a single tank. This configuration makes it possible to better cope with the potential variations in load, for example in high tourism areas. It also makes it possible to maintain certain reactors on standby with very short aeration phases when the total treatment capacity of the installation is not required and it enables high treatment capacity during peak periods.

Emergency pumps

• Throughout the system, secondary pumps can be activated to increase admitted flow rates (without exceeding the capacity of the downstream system). They also function as a backup and take over if the main pump fails.

Non-return valve

• Equip the outfall to the natural environment with a non-return valve to prevent water from entering the plant in the event of flooding at the discharge outfall.

4.5.7 Recommendations - Biological & Physicochemical Parameters

Ventilation

• Adapt the air flow rate to the volumes to be ventilated;
• Avoid dead zones and deposits by suitable diffuser arrangement;
• Include a de-scaling reagent injection port on air blowers;
• Avoid under-ventilation or prolonged stoppage of ventilation (more than 2 hours), which may increase overflow risk;
• Adapt the number of dissolved oxygen sensors to the structure and flow rate of the system;
• Double the oxygen probes to guarantee measurements in case of electromechanical failure or obstruction by sludge or foam;
• Provide a degassing area between the biological reactor and the clarifier to facilitate the removal of air bubbles and improve the recovery of extractable sludge.

Mixing

• Ensure ideal mechanical mixing by optimising the positioning of the agitators in the tank (angle, immersion height, etc.) and by eliminating obstacles (lateral guides for the diffusers, transverse channels, etc.)
• Keep a backup agitator in case of electromechanical failure of the main system.

4.5.8 Recommendations - Retention Equipment

Design closed tanks for new construction

• In the case of an upgrade of an existing installation, all outfalls and basins containing biocarriers should be equipped with grids with a mesh size smaller than their diameter.
• Installing other gooseneck- and/or non-return valves on effluent inflow pipes could also prevent backflow circulation of biocarriers in the case of water level rise.
• In the case of a new system, the inflow could be directly poured from the top (upper dale) of the tank, top-down oriented, to prevent biocarriers from entering the pipes. This type of installation requires less maintenance than grids/mesh, which have a higher tendency to clog.

Retention Screens

• Ensure that all inlet and outlet streams of reactors containing biocarriers are equipped with retention systems (screens, check valves, goosenecks, etc.) with mesh sizes selected according to the biocarrier geometry
• Equip all openings with physical barriers to prevent the passage of biocarriers.

• Add strainers on wastewater pump flow leading to the MBBR to prevent the transfer of biocarriers from the reactor to the lift station in the event of accidental pump reversal.
•  Equip each of the following circuits in the same way:
  • Exhaust air
  • Reagent addition hoses
  • Sludge disposal system

• Add multiple screws or weld the entire length of the protective equipment rather than at a few attachment points that could cause structural weaknesses and breakage, allowing the biocarriers to escape.
• Install filters or collection grids at all surface water drainage outlets.

This should enable recovery of biocarriers in stormwater in the event of spillage on the ground within the WWTP. These screens should be easy to check and maintain to avoid clogging.

4.5.9 Recommendations - Prevention of clogging

Unclogging mechanism

• Inject water or air under pressure onto or through the grilles to prevent clogging. This operation can be programmed automatically for regular grill maintenance.

Special precautions when upgrading an existing WWTP

• The protective elements should be adapted to avoid the risk of clogging due to the accumulation of biocarriers on their surface. Specifically, flat grids should be replaced by cylindrical grids. 

4.5.10 Recommendations - Securing Electromechanical Equipment

Power supply network

• Ensure the doubling of the supply networks at each station. A fault detection system is imperative.

Control-command system

• Install a backup system to ensure the two principal functions of the control system: to coordinate all the equipment in the treatment plant and to monitor the equipment’s operational capacity by emitting alarms in the event of a malfunction. 

Reagent injection station

• Ensure that visual verification of correct sensor function and reagent stock levels is possible.

• Enable manual addition of reagents, supply of dilution water, and homogenisation of the effluent in the event of a motor or valve failure.

In the biological treatment tank

• Double the equipment in critical areas to protect against failure of the main sensors. 
• Equip biological tanks with level sensors

• Connect the water level sensors to a system that shuts off the effluent supply to the tank. 
•  Implement a multiple alarm system (visual, audible, sending SMS-type messages) to warn of malfunction as soon as possible.
•  Install cameras to check for blockage of screens or probes in closed tanks inaccessible by staff.  
• Install an alert system to warn of a malfunction when no personnel is on site. The alert should be able to send automatic notifications to the phone of an on-call operator or to a central office that controls several installations.

In the clarifier

• Install an alarm system for extraction pump malfunction, measuring the volume and turbidity of the sludge at the clarifier outlet to ensure it is properly removed and does not spill back into the aeration tank.

4.5.11 Recommendations - Means of System Maintenance

• Provide an access hatch for closed tanks
•  Provide a control opening that is always functional and above water to allow for visual inspection the biocarrier mix, sampling to monitor adequate colonisation of the biocarriers, and to facilitate cleaning operations.
•  Facilitate cleaning operations
  • Install guide bars and jibs equipped with winches or hoists to raise the often-heavy equipment installed at the bottom of the pools (baskets, pumps, etc.).
  • Provide cleaning areas for equipment that require regular maintenance or changing (pumping device strainers, grids, etc.) 
  • Provide immediately operable emergency equipment to deal with any accidental shutdown.
• Ensure access to treatment basins and maintenance points at all times of the year, especially in snowy areas.

4.5.12 Recommendations – Instructions for Safe Operations

Creation of a good-practice guide

• Preliminary audits
• Terms and conditions of use
• Storage conditions
• Handling and introduction into tanks
• Overflow protection
• Appropriate retention systems
• Aeration and parameters
• Processes for monitoring the effectiveness of the system
• Self-monitoring protocol
• Key maintenance operations

Train operators

• Plan training sessions to ensure that operators stay up to date and that good practices specific to biocarriers are understood.

4.6 WWTP Start-Up Phase

4.6.1 Existing Measures and Reported Weaknesses

4.6.2 Recommendations – Establishment

Framing of the start-up and stakeholder involvement

• Commissioning should be carried out in strict accordance with the procedure proposed by the installer, who should be present and accompany the operator.
•  Recommendations for use should be provided by the treatment plant designers to help operators run the plant more safely and include biocarrier-related specificities. (cf 5.5.10)

• Quality control of facilities
• Storage
• Handling
• Operating parameters
• Maintenance

• The installer should be present in the weeks and months following commissioning (according to a set schedule) to monitor the ramp-up and optimal functioning of the WWTP.
•  Start-up phase engineers and operators should have a detailed knowledge of the HIRA. They should check the correct functioning of the control points and upgrade them if necessary.
•  Regulatory authorities should be informed at the time of impoundment and final acceptance of the facility.
• Test phase

Requirements before the start-up period

• Ensure that no chemical pollutants are present in the effluent that could deteriorate the plastic materials.
• Check that all drains and outlets are protected by screens, grates, or cages with a suitable mesh size to prevent biocarriers from passing through.
• Check that the biocarriers are not in contact with installations (pumps, mixers, etc.) that could accelerate wear.
• One month before the planned commissioning date, a start-up review should be carried out to ensure that the equipment is ready. This involves:
  •  Performing leakage tests on tanks and pipes;
  • Carrying out initial tests and trials of the pumping systems;
  • Checking electrical connections;
  • Carry out electromechanical tests on the hydraulic, aeraulic, and pneumatic installations in order to check their proper functioning (check the power supplies and the correct direction of rotation of the motors, test all the station’s sensors);
  • Check, under the supervision of the automation engineer, the automation and the correct linkages between the instrumentation and the equipment.

During the start-up phase

• The commissioning period should include:
  • Setting up the facilities;
  • Verification of the proper functioning of all mechanical, electrical, electronic, thermal, and hydraulic equipment in accordance with the relevant standards and the operating conditions stipulated in the contract;
  • Drafting of operating instructions and training of the operating personnel who will be responsible for the installations;
  • Progressive input of effluent with the measurements and analysis necessary to assess the flow and load received at the plant;
  • Close monitoring to quickly detect possible failures;
  • A performance test to certify the compliance of the system with its stated objectives and the functioning of the MBBR biological treatmen;

Introduction of biocarriers into tanks

• Pouring biocarriers carrefully

• Determining the optimal volume of biocarriers to introduce into the tank

Colonisation

• Stirring biocarriers

• Optimising the physical parameters of effluents

• Starting with external sludge

• Mud-free starting
  •  With clear water: water can be taken directly from the natural environment (freshwater, free of sediments). The filling time of the pond should not exceed a few days with active aeration during the entire process.
  • With wastewater: wastewater is only admitted to the aeration tank after it has passed through pre-treatment. The solution is often to promote the start of the flocculation mechanism of the bacteria by limiting the load to be treated to reduce the mass load and facilitate flocculation (transient bypass) or by adding coagulation-flocculation reagents.

Observation period

Acceptability of the work

• Proper removal of construction equipment and restoration of the site;
• Validation of the results of the tests, with corresponding trials and performance checks.

Handover and defects liability warranty

4.7 Operation of the Wastewater Treatment Plant

4.7.1 Existing Measures and Reported Weaknesses

• Effluent composition and its variations
• Mud levels
• Aeration/mixing rate
• Development of filamentous bacteria which can lead to the deterioration of sludge: non-settling or over production
• Infiltration, deposits, heterogeneity, grease
• The advent of foams trapping biocarriers at the surface and disrupting level probes

• Uninformed or insufficiently-trained operators on the specificities of biocarrier processes.
• Non-anticipation of the need for rapid intervention in the case of bad weather forecasts.
• Bacterial imbalances leading to the development of filamentous bacteria and foaming.

• poor sludge settling due to increased sludge volume.
• formation of a thick layer of floating material on the surface, which in turn provides a favourable environment for the development of filamentous bacteria.

• a rapid increase in load with a punctual and massive input of organic substrates (industrial effluents, sewage, etc.) causing a relative lack of oxygen (the immediate need is not satisfied even if the overall oxygen input seems correct),
•  the arrival of a large amount of a toxic product causing the destruction of a large part of the biomass and leading to a loss of installation performance.

• Neglected maintenance of equipment preventing biocarrier loss;
• Obsolete maintenance plans and non-reported incidents;
• Unanticipated requirement of spare parts.

4.7.2 Recommendations - Nature of the Effluent

Ensuring the quality of effluents

Limit the introduction of salts into the network

Limiting the concentration of sulphur

• Increasing the flow rate in collection networks to limit sulphide levels

• Regularly check lift stations

Monitor non-domestic discharges to sewer systems

4.7.3 Recommendations – Prevention of Foaming and the Development of Filamentous Bacteria

Preventive measures

• Adapting aeration requirements to the nature of the effluent
  • Have a surplus oxygen production capacity available to cover peak demand, combined with a good oxygen measurement system in the different parts of the tank to ensure optimal aeration (based on the installer's recommendations and possibly modified according to the operator’s observations);
  • Ensure that the ventilation system continuously and evenly agitates the biocarriers;
  • If the oxygen requirements of the biomass are greater than the agitation requirements, the air supply must be adjusted to the biomass requirements;
  • Otherwise, to save energy, it is possible to limit the ventilation input to the biomass needs and add mechanical ventilation.
• Check the proper functioning of the mixers
  •  Check the effective rotation of the blades and their direction of rotation by detecting hydraulic movements or by reassembling the unit if necessary (checking for blade wear and dross);

    Groupement d'Intérêt Scientifique (GIS) Biostep, 2013, Dysfonctionnements biologiques, Pourquoi et comment vérifier le bon fonctionnement d’un agitateur ?
  • Check the power consumption of the agitator for optimum operation and the presence of dirt. The measured power should correspond with the manufacturer's power setting;
  • Check the mixing at different points in the tanks to ensure that the desired objective is achieved and that there are no zones of sludge accumulation;
• Ensuring mass load stability
  • Ensure that the optimum mass loading is stable (ratio of the mass of oxygen to be supplied over the mass of microorganisms in the aeration tank).
•  Monitor the temperature of the effluent
  • The temperature of the sludge or effluent affects the treatment and can cause malfunctions. Therefore, care should be taken to maintain the temperature range for the optimum development of the purifying bacteria.

Curative actions

• Removing floats
  • The removal of floats from the biological reactors should avoid recirculation and permanent reseeding.
• Ballasting of effluents
  • This technique is based on the addition of a high-density substance, usually mineral (talc, ash, calcium carbonates, metallic salts, etc.), which, when added to the bacterial floc, improves its settling. Ballasting is useful for reconstituting a bacterial floc following major sludge losses. Ballasting of the floc can also be achieved by adding sludge from a neighbouring treatment plant with a good sludge index or by bypassing part of the water in the primary stage to ballast the floc with the particulate fraction of the raw water.
• Adding oxidants
  • Use of chemical agents (oxidants) that have a bactericidal action on the sludge. Their addition leads to a modification of the structure of the sludge by breaking the filaments. It should be noted that the results obtained cannot be definitive because these products do not act on the causes of overflow.

    Cailleux, G., 2001, L'épuration biologique: fonctionnements et dysfonctionnements. Dissertation. D.E.S.S. " Qualité et Gestion de l'Eau ", Fac. Sciences, Univ. Picardie Jules Verne, 63 p.
• Managing large load variations
  • Injection of a nitrogen substrate during a sudden increase in load should increase the usually slow-growing nitrifying biomass.

4.7.4 Recommendations – Limitation of Sludge Overabundance

• Ensure the proper functioning of the turbidity and sludge flow sensors to monitor the sludge concentration in the aeration tank;
• Smooth out the hydraulic load and control the recirculation of sludge;
• Ensure proper operation of the clarifier scraper arm.

4.7.5 Recommendations - Maintenance

Carry out preventive maintenance

• A calendar indicating the dates and frequency of maintenance of the main electromechanical parts (motors, etc.) and protection systems (grids, nets, cages, etc.);
• A datasheet for each piece of equipment, indicating its characteristics, but also the contact details of the manufacturer and suppliers;
• Spare parts for all equipment classified as having a high risk of biocarrier loss in the event of an incident (pumps, grids, etc.).
• The essential tools, spare parts, and consumables subject to frequent replacement (screws, repair tools).
• Appropriate tools (shovel, rigid container, industrial vacuum, etc.) to recover biocarriers before they reach stormwater or wastewater outfalls, the surrounding environment, or waterways.
  

Establish a maintenance contract

Implementing a computer-aided maintenance management system (CMMS)

• Regularly check several parameters
  • Sensors in the collection network and the WWTP (O2, H2S, water level, ...);
  • Presence of foam;
  • Correct positioning of grids and meshes;
  • No clogging;
  • No biocarrier outside the tanks.
• The data from this self-monitoring, including the monitoring of biocarriers, should be transmitted to the authorities responsible for monitoring the treatment plants concerned on a regular basis.
  

Additional measures in case of incidents

• Determine the causes and effects of the failure
• Define the severity, occurrence, and detectability of the failure
• Define corrective solutions (preventive, corrective, complementary, curative, palliative measures, emergency spare parts to be stored)
• Recalculating the risk index
• Assess the quality of the crisis management plan, readjusting it if necessary, in the light of recent events.
• Rectify maintenance protocols and frequency to prevent the recurrence of the failure.

4.7.6 Recommendations - Operator Training

Organise training sessions

• Grille and other protective device maintenance
• Identification of failures that can lead to biocarrier losses and corresponding response measures
• Alert and recovery protocols

4.8 Installation Monitoring

4.8.1 Existing Measures and Reported Weaknesses

• Self-monitoring requirements do not include reporting of biocarrier loss or the status of equipment preventing biocarrier loss;
• The frequency of monitoring may be too low to detect failures;
• Generally, regulatory agencies are not trained in biocarrier processes and the following specific control points are not the subject of oversight:
  • Biocarrier loss within the plant, at the WWTP outlet, or in sludge
  • Presence of protective measures (grilles...)
  • Emergency plans in case of biocarrier leakage in the environment

4.8.2 Recommendations - Monitoring by the Project Owner

Implementing self-monitoring involving biocarriers

• Self-monitoring should be based on the Hazard Identification & Risk Analysis.

Train the operator to implement self-monitoring.

Self-monitoring reports

• On an annual basis self-monitoring reports including biocarriers related hazard history should be transmitted to regulatory agencies.

4.8.3 Recommendations - Regulatory Agency Oversight

Regulatory agent training

National database

• Enter technologies used for biological wastewater treatment in national WWTP databases and update them regularly with a supranational Agency such as the European Environment Agency.

• Minimum information to be provided:
  • Installer and operator of the WWTP;
  • Type of technology used (MBBR, IFAS, ...) ;
  • Start date;
  • Type of biocarrier;
  • Quantity introduced ;
  • History of biocarrier-related malfunctions.

Monitoring programmes

• Arrival of wastewater;
• Aeration and good mixing within the biological treatment;
• Sensors (O2, water level...);
• Retention measures (presence of grids);
• The durability of materials;
• The structure of the tanks (materials);
• Pumps (sizing/backup pump...).

Environmental monitoring protocol

4.9 Emergency Plans

4.9.1 Reported Weaknesses

• Local authorities and municipalities are not informed of possible risks of pollution induced by WWTPs using biocarriers on their territories.
• There is no clear plan of action in case of leakage of biocarriers into the environment.
• Companies and NGOs capable of responding to pollution incidents are not identified in advance and are not contacted in time.
• Operators tend to underestimate, or even hide, the impact of leaks thus delaying the adoption and implementation of appropriate measures.
• When they do occur, clean-up actions are often launched too late due to the rapid spread of plastic carriers in river and marine environments.

4.9.2 Recommendations

Definition and implementation of the crisis management plan

• Have a crisis management plan that includes the risks posed by biocarrier leakage

• Contacting stakeholders to help reduce biocarrier dispersion:
  • Local municipalities;
  • Local water agencies;
  • Local environmental NGOs;
  • Remediation companies;
  • Representatives of the press (to inform riverside communities and citizens).
• Listing existing containment and recovery solutions
• Establishing an appropriate recovery plan for biocarriers dispersed in the environment, including:
  • The installation of booms and stop nets in the water column;
  • The development of a dedicated website that allows citizens to report the presence of biocarriers in the environment (date, location, quantity, with the possibility of uploading photos), allowing recovery operations on the main accumulation areas;
  • The possibility of partnering with local NGOs that have experience in organising and managing waste collection.

• Mapping of nearby rivers and water bodies and their catchment areas. Identify existing dams and potential areas for temporary dams as well as natural areas for biocarrier accumulation.
• Identifying a closed storage site for the collected biocarriers to prevent supplementary incidents and identify a reprocessing channel.
• Allocating a budget for immediate mobilisation in the event of a crisis.
• Display the crisis management plan in the treatment plant in a visible manner, in sufficient size and quality, including contact information for operators who can answer questions on-site.
• Make essential instructions available in the vicinity of recovery facilities and equipment.
• Present the crisis management plan to the local authorities.
• Inform stakeholders of developments in the event of pollution.
• Monitor the weather daily, to prevent possible supplementary incidents.

Update of the crisis management plan

• Carry out an annual inspection of:
• The relevance and validity of the crisis management plan after possible changes in the operation of installations and equipment;
• The validity of the reference persons' contacts;
• Presence and conformity of informational posters;
• Presence and condition of containment and cleaning equipment.

4.10 Environmental Responsibility

4.10.1 Reported Weaknesses

4.10.2 Recommendations – Polluter Pays Principle