CONTINUOUS CLEANING OF AN ANAEROBIC DIGESTER
The AMP® technical team reports on the field trials, with a pilot operation at a municipal Waste Water Treatment Plant (WWTP), for the disposal of inert material using continuous anaerobic digester technology by solid-liquid separation, thereby saving costs and time, together with occupational risk minimization.
Initially, the objectives sought to be achieved with the operation of the hydrocyclone pilot plant were:
- Disposal of inert matter:
- Increasing digester performance.
- Increase in the production of biogas.
- Increased cogeneration energy.
- Removal of mineral crystals:
- Minimization of abrasion in centrifuges.
- Decrease of blockages in pipes.
- Decrease in the use of chemical agents
- D1: Regulation reservoir tank.
- B1: Feed pump to the hydrocyclone plant.
- D2: Sump.
- D3: Recirculated pulp tank.
- B2: Pulp recirculation pump.
- E1: Sector of vibrating screen for pre-treatment of the pulp from the digester.
- E2: Sector of vibrating screen for dewatering the discharge from the hydrocyclones.
- 1. Input to the plant, after the feed pump.
- 2. Mixing or hydrocyclone feed in central sump.
- 3. Output of hydrocyclones from upper nozzle (overflow).
- 4. Output of hydrocyclones from discharge spigot (underflow).
- 5. Sample from vibrating screen (pre-treatment)
- 6. Sample of fine vibrating screen discharge
- After 10 months of operation, with an average of 10 hours daily operation, the volume of waste from the digester was reduced from 6.04% to 2.66%, representing a recovery of actual usable volume of more than 50% of initial waste volume
- The process used allowed for removal of inorganic and volatile matter (the latter composed mainly of fibres and vegetable matter) enabling a non stop process of anaerobic digestion.
- From the results of XRD tests it was concluded that the major minerals obtained as final residue, once treated by the hydrocyclone were quartz and calcite, discarding with struvite (possibly because of the absence of this mineral, prior to the hydrocycloning treatment).
- According to historical data of biogas production in recent years, after 10 months of activity an increased of about 30% was detected, without knowing whether this increase is directly associated with this treatment process or other actions taken on the digestion process.
- We can conclude that the continuous cleaning of anaerobic digesters through a process of hydrocycloning may be proposed as a viable alternative to conventional cleaning anaerobic digesters, especially in installations where there is a unique element of the sludge
Given that it is a full-scale plant, with resultant data obtained being easily extrapolated on an industrial scale, we proceeded to evaluate the efficiency of the method selected by conducting a preliminary study of the current working volume of the digester, the objective being to reduce the volume of waste from the digestion reactor.
With information on the material to be treated known, and according to our experience in the classification of solids by hydrocycloning, we defined the most appropriate configuration of the hydrocyclone plant in order to achieve the stated objectives
The Hydroset Compact Hydrocyclone plant consists of three main modular components - a pumping set, a hydrocyclone assembly and a dewatering screen
The main function of the Hydrovortex Hydrocyclone is to separate the suspended solids in a slurry feed into two fractions, the "underflow" carrying suspended coarse solids above the predetermined size of cut, and the "overflow" containing finer size solids in suspension.
The working principle of the Hydrovortex hydrocyclone is shown in the diagram, which shows how the feed slurry enters the cylindrical section tangentially under a predetermined pressure causing it to swirl around the longitudinal axis of the hydrocyclone and descend toward the cone apex section. With the centrifugal forces generated, the coarser particles rotate near the wall and are evacuated through the apex cone in the form of thick slurry. As the apex is small in size, only part of the suspension is evacuated, while a secondary swirling flow is created in the core of the cone and moves upward, transporting the fine particles together with most of the liquid. This suspension is discharged through a central pipe located in the upper end of the hydrocyclone. By regulating the acceleration of the swirl and varying the hydrocyclone geometry and nozzle dimensions, the cut-point can be adjusted
The Hydroset Compact Hydrocyclone plant selected for these trials operated as follows:
At the hydrocycloning stage, different sizes and configurations of hydrocyclones were studied in order to assess the wide range of settings for each type of hydrocyclone tested, in order to determine the exact type and configuration necessary, based on the characteristics of the pulp, to provide maximum data and identify the best operating conditions.
For pumping the pulp to the hydrocycloning stage a centrifugal pump, inverter equipped with an inverter variable speed control and installed power of 15kW, was incorporated
To carry out the first stage of classification of pulp from the digester, as well as for the treatment of the hydrocyclone discharge, a high frequency/low amplitude vibrating screen was used, driven by out-of-balance vibrators with a total power of 3.2kW.
A combination of different screen media types and configuration of the vibrating screen achieved the correct solution, rejecting unwanted materials and delivering solids with low moisture content.
The circuit diagram of the equipment installed in the municipal WWTP for conducting the tests is shown below:
The pulp from the digester is stored in a pre-plant buffer tank (D1). From this surge tank, a centrifugal pump feeds to the vibrating screen (E1) to for pre-treatment to remove the coarser fraction. The pulp passing through the screen, containing most of the water and solid fines in suspension, is also pumped to the stage of hydrocycloning by a centrifugal pump. The overflow containing the fine fraction is returned to the digester with the settled fines for dewatering in the main sector of the screen (E2). The entire facility was equipped with appropriate automation and control systems to enable the trial work to be undertaken with minimal staff assistance.
During the period that the plant was operating at the municipal WWTP daily systematic analyses of dry and volatile matter were conducted at various points to monitor and control the plant performance. Mud samples were taken at the following points:
Finally, to test the efficiency of cleaning plant hydrocycloning on the actual volume of the digester, a second set of tests was conducted after 10 months of operation, yielding very favourable results.
The most representative findings that were obtained are extracted as follows: