Integrating a media bed with a DWC or raft type system has a multitude of benefits. One mustn’t only consider the increased growing diversity, but the additional nutrient availability, the addition of a tremendous biofilter, the reduction of labor from not removing solids and the benefit of utilizing those solids. Of course if this is to be considered for a commercial application, the additional cost of integrating a media bed must be considered as it will obviously incur a somewhat larger price tag then a DWC system alone. However the gain of integrating the two types of systems easily justifies the increased build cost.
Deep water culture or raft type culture undoubtedly has it’s benefits in commercial type growing ventures. It can produce certain crops with relatively less labor then a media system; however the market is a tremendous consideration if DWC alone is going to be sufficient. Monocropping or growing thousands of pounds of lettuce for large grocers or big box stores demands highly efficient growing systems where large, almost automated type production is paramount. How many aquaponic ventures intend to monocrop lettuce though? Or do they intend to thrive as a small family farm catering and bolstering a local economy. If a market is secured that is linked to a local community and catering to local restaurants and chefs, farmers markets, health food stores and buying clubs then all of these outlets demand one common denominator, product diversity. Therein lies the problem with DWC by itself.
Crops of the woody variety, peppers, eggplants, large producer tomatoes and even spinach tend not to perform well in raft culture. That is just a few examples and there are several others that rafts can just not support because of the root structure needing something more substantive to grab hold of and they also require higher levels of potassium and phosphorus than a DWC system can supply. There is also the concern that a media system would incur greater labor inputs with planting and therefore not making media conducive for high volume turn over. That is absolutely correct, but only for high volume turn-over plants such as lettuce. With a hybridized system developed to optimize growing diversity, one would never plant lettuces that would be harvested out in three to four weeks into the media bed. For that matter, any plants that are expected to have a relatively short span in the system would be kept in the rafts where planting is simple and can be facilitated in a highly effective manner right from the proximity of a seedling area. Labor would place lettuce and other comparable type seedlings into rafts and merrily float them down the deep water troughs where they will live out their relatively short life in the system. Whereas, producer type plants that would be expected to have a dramatically longer life span in the aquaponic system would be planted into the media bed eliminating the concern that planting in this type of system is too labor intensive as it is only actually done a fraction of the time. Harvesting would then include picking fruits and vegetables over a long time span and therefore not have any greater labor impact. Also, the time spent disposing of used media and the washing of net pots is also eliminated.
Nutrient availability from the additional mineralization is another tremendous asset of this type of system integration. The inherent nature of media bed systems is that they support greater levels of the minerals that fruiting plants require to be productive such as potassium, phosphorus and calcium from the mineralization that occurs from the utilization of the solid waste. There is typically little to no provision for the same type of mineralization in DWC alone. This additional mineralization will also aid in maintaining much lower stocking densities. This can be incredibly important if the fish commodity does not bring a good return, as it may necessitate maintaining lower stocking densities. Then the objective is to maintain as little fish as possible, minimizing the associated costs while optimizing plant growth. It’s a simple equation; less in, more out. The expenses associated with cultivating the fish/fertilizer generating side can be significant in feed and electric inputs for large populations. Optimally then, an efficient farm will operate on the ratio that allows maximum plant growth with as little fish as possible and view the fish as nothing more than an occasional treat at dinner time for the farmer and the cost of producing his primary stream of revenue, the produce.
When considering added value to the actual system operation, there is the elimination of the cleaning and removal of the solids from a settling tank. Anyone that has ever had to remove solids from a settling tank will probably agree that it is the least likeable part of operating a large system. Removing these solids often is essential, as even at low densities, the buildup of solids over time in these systems is problematic on various levels. Depending then on system size, fish density, feed ratios and efficiency of the solids tanks, these systems must be cleaned anywhere from daily to weekly or bi-weekly and can take from a few hours to the better part of a day. Long term thriving media beds with red worm populations have shown to operate upwards of 4 to 5 years before requiring any cleaning. Weekly as opposed to once every 4 to 5 years is a huge labor savings and a tremendous plus for overall operator happiness! Besides why would anyone want to remove such a valuable part of the system?
Perhaps the most important consideration for the business owner is the additional cost of including the media bed as opposed to the lesser cost of just a DWC build out. Is it worth the additional investment? If you were to consider the same size media bed and DWC setups side by side, the numbers could easily scare any investor, however that is not the nature of the integrated system. What must be considered is the additional costs to include a media bed with a DWC set up. It will obviously be more then the comparative cost of a DWC system alone, but in order to make a fair and reasonable comparison, only certain factors should be included. Both systems will have the costs associated with the infrastructure of the bed itself, regardless if it is DWC or media, so those factors should not be part of the equation. The differences therefore lie in two places, the additional cost of the construction to support the media bed weight and the cost of the media itself. However, don’t forget to remove the substantial cost of the raft material from the equation and the airlines, diffusers, net pots and growing media which is actually an ongoing expense in the comparative raft trough. When the costs are realistically and accurately compared, the media bed cost is only slightly more. For example, the additional cost to build an integrated system that would include 200sqft of media bed and 800sqft of DWC would only be $2800 more than constructing a comparable 1000sqft DWC system. This is merely a 17% increase and it is calculated with the most expensive media available, so that additional cost could be as much as $800-1000 less depending on the type of media selected.
This additional cost of anywhere from $1800 to $2800 more to build an integrated media/DWC system will easily be worth this investment based upon the ability to efficiently grow crops such as large heirloom tomatoes or red, orange or yellow bell peppers or heirloom striped eggplant. Even when in season, these tasty veggies and fruits bring a premium price and require no processing or special packaging. Of course a DWC system may be able to grow these things, but when growing commercially, the goal is optimization and integrating a media bed optimizes so many aspects for the small family farm, the slight increase in initial investment is well worth it.